Converting Pathfinder and D&D Monsters to the Cypher System

Okay, I can't hide it: I am loving the Cypher System. This is a modern system that manages to capture an incredibly easy feel and design to it which is something I just don't see in a lot of contemporary systems, even while being a robust ruleset with lots of stuff for players to mess with. Despite having lots of monsters and three bestiaries out (for The Strange and Numenera), it's still weak on fantasy monsters for Cypher System Fantasy Gaming, so I thought I'd try out making conversion guidelines for converting Pathfinder Monsters over to Cypher System. These rules could easily work for D&D 5E as well, and I'll provide notes for such below. I'll be testing this system out with some monsters in the Saturday game. 

Converting Pathfinder Monsters to the Cypher System

Level

The simplest expression for the Cypher System is the monster level. Luckily it aligns fairly well with hit dice/level in D20 system games with some modification. I find this process below to get the best comparable results, but you might want to fudge it if the result feels over or under powered. 

Simplest conversion for this is to simply take the creature’s hit dice, and divide by 2. After level 8 move to 3 Pathfinder HD or levels per 1 Cypher level. At level 21, you might use your best judgement or the suggested chart below:

Pathfinder Hit Dice (or level)     Cypher System Level              Target Number

1-2                                                               1                                       3

3-4                                                               2                                       6

5-6                                                               3                                       9

7-8                                                               4                                       12

9-11                                                             5                                       15

12-14                                                           6                                       18

15-17                                                           7                                       21

18-20                                                           8                                       24

21-30                                                           9                                       27                                                  

31+                                                              10                                     30

Use this same process for converting HD/level in D&D 5E to Cypher System.

Health

Health should normally equal the target number, but a creature with a large health bonus should be X4, and a huge or larger creature should have X5-X6 or greater as fits the type.

Damage

Quick Conversion: damage equals level determined above, as per Cypher System default rules. Use the other system's description of the attack to determine how it deals the damage, but use the damage set by level as default. If the creature deals a lot more damage than normal, or less, adjust up or down a point or two accordingly.

Detailed Conversions of damage can probably be done, but it's tough because the damage/health mechanic in Cypher System does not correlate closely to how D20 mechanics deal damage. My suggestion is as follows:

Most Pathfinder creatures can fit damage expressions in to Cypher’s 2/4/6 structure neatly by eyeballing whether the attacks sounds light, medium or heavy, but special attacks may do 2 or more extra damage. A suggested conversion is: determine average damage from a primary common attack, and set it at 2 damage for every 8 points the creature deals from the average of a single strike. Attacks that do 32 or more in a single strike should have a damage bonus of 2 or more for every 20 extra points that triggers on a potentially avoidable secondary effect.

D&D 5E monsters often do considerably more damage, so it's best to use damage appropriate to the weapon type, or do 2 damage for every 15 points on average. 

EDIT: an alternative I was thinking about is to set the level determined earlier as the base damage. Now, that value X3 is the maximum damage dealt in a combat round. If the damage dealt in Pathfinder or D&D exceeds that value, then add 1 damage for every full 12 points of extra damage dealt in PF/D&D. Example: the Gashodokuro breath attack deals like 12D6 or up to 72 damage, which when compared to its level damage of 8 means you could increase this to 12 damage for the effect if you wanted the attack to be deadlier.

Save Effects

You can divide Pathfinder Saves by 3 and round down to determine the level for an appropriate defense roll; more often than not these will match the actual level of the creature. In D&D 5E you can do the same, but creatures will have lower average saves, so use either the level you determined by HD or this result, whichever is better.

Special Abilities

Most special abilities will be hard effects that trigger similar results in Cypher System, substituting the appropriate mechanical result. Effects which clearly deal mental or psychic damage should deal damage to Intellect first. Special abilities which only pop once could work as GM Intrusions, instead.

Armor

Armor Class makes a creature harder to hit, so should reflect a greater (or lesser) modifier to the Level generated target number for attacks. Take the AC and subtract 10. If the remaining AC is greater than the hit dice, it means the calculated level should be 1 higher for defense purposes for every 10 points of AC over the hit dice. If the AC is lower than the hit dice by more than 5, then the defense level is lower by 1 point for every ten points under. Example: the Gashadokuro has 19 HD and AC 28. It would be Level 8 but Level 9 for its defense against attacks.

D&D 5E AC expressions are much lower; do as above, but lower or raise Defense level for every 5 points of difference between the AC (after subtracting 10) and the HD.

Damage Resistance

Damage resistance is a different story. Any DR at all adds 1 point of armor. DR of 11+ should add 2 points of armor. This armor should be noted as bypassable if an appropriate attack type is used (if indicated). For tougher creatures, add 1 point of armor per 5 points of DR.

D&D 5E only has Damage Resistance. This can be converted straight over as reducing damage dealt by 1/2 of the appropriate closest type, or you can assume armor of 1 against that type, or armor 2 if level 10+, or higher if abilities support it.

Distances

This is easy....if you need to be within 5 feet to make the thing happen, it's immediate range. Anything from 10 feet to 50 is short range in Cypher System, and anything from 50-100 feet is long range; use the specified distance if its farther than that.

GM Intrusions for Creatures

Creatures with especially distinct or one-use abilities might be suited treating this unique ability as an intrusion.

Example Conversions: 

These are all from the Pathfinder Bestiaries, but I'll try 5E monsters out soon as well.

Gashodokuro (B4, 121) Level 8 (24)

This is a huge 30 foot tall undead being formed from the fused skeletons of hundreds of starvation victims that continues to suffer in unlife, hunting the living.

Health: 24

Defense: Level 8 (24) against attacks.

Armor: With its impressive damage resistance it gains armor points of 2.

Attack: giant pounding fists for 6 points.

Starvation Aura (Level 8): This is a long range aura in radius and all in the area, on the start of their turns, roll against Brawn or suffer 2 damage.

Corpse Breath (Level 8): a short range cone of bone shards, dealing 8 damage, dealt once every 3 rounds.

Corpse Consumption: A slain foe will be devoured by the gashodokuro which will regain health equal to the unmodified Might Pool of the creature before death. This does not give more health than the creature started with.

GM Intrusion: the gashodokuro immediately unleashes its corpse breath, even if has not regenerated yet.

Hellcat (B2, 153) Level 5 (15)

Voracious and intelligent predators from hell!

Health: 15

Defense: Level 5 (15) but see it's invisibility below..

Armor: The hellcat has armor of 1 point and 2 points from fire and heat attacks, but creatures of a good nature can bypass this (angels, magic weapons and cyphers of a certain moral alignment, etc.).

Damage: bite and claws for 5 points.

Invisible in Light (Level 5): Hellcats also are invisible when in full light. In dim light they become partially visible, and in darkness fully visible with their hellish flames. A test to spot them in full light is level 5 (15) to succeed. An inivisble hellcat in daylight is level 7 on its attacks and defenses, level 6 in dim light, and normal (level 5) in total darkness when its flames of hell are visible.

Silent but Intelligent: Hellcats understand the infernal language but cannot speak. They can communicate with intelligent creatures nearby telepathically.

GM Intrusion: The next attack is a Pounce and Rake attack (Level 5) immediately dealing 6 damage and knocking the enemy prone if successful.

Catfolk (B3, 47) Level 1 (3)

Tribal hunter gatherers of the woods who are in harmony with nature (at times).

Health: 3 but a warrior catfolk may be level 2 (6) and health 6 or greater.

Defense: fast and nimble, they are level 2 for defense.

Armor: none but warriors may wear light leather armor for 1 point of protection.

Damage: 1 point claws, but also typically a longsword or longbow (4 points), but catfolk specialize in hunting magical beasts and may do +2 damage as a GM Intrusion to beings of a magical nature they attack.

Cat’s Luck: once per encounter the catfolk can call upon this ability for an attack or defense. The PC must roll twice and take the lower of the two results.

Sprinter: catfolk are fast and can take a move and regular action in one round.

GM Intrusion: The catfolk gains an edge due to its speed and the PC must make a Level 4 (12) intellect test or lose track of the catfolk, which either disappears or may make a stealth attack (at level 4) on its next round.

Bonus!

Catfolk as a Decriptor for Characters

I’m using catfolk in my Ensaria campaign, as a race of wild southern tribal groups who migrated into the Zankani woods centuries ago, and lived in harmony with the men of Ensaria for centuries until invaders from the west disrupted the region, leading to decades of civil war. The catfolk are attuned to nature, worship the deity Susuram the Lord of the Golden Lotus in a quasi-shinto system of belief with deeply animistic roots, and are extremely tribal.

Swift: +2 to your Speed Pool

Cat’s Luck: Once per day you may call upon cat’s luck to roll two D20s and pick the best of the two (costs 1 Intellect point).

Swift: You are trained in Nature tasks related to agriculture, herbalism, animal husbandry and game hunting.

Skill: you are practiced in using bows and swords.

Skill: you are trained in hunting and tracking and can use your enhanced sense of smell for such.

Slender Build: when you fail a Might roll to avoid damage, you take 1 extra damage.

Equipment: you may choose a longsword or longbow (4 point weapons, medium).

Suggested Links to the Starting Adventure:

1.       You met the party while out hunting a local beast and aided them in a skirmish.

2.       You are on a quest to determine your status and purpose for the clan and have decided to take the wanderer’s path.

3.       You owe a debt to one of your companions who saved your life while you visited the human city.

4.       In a vision revealed by the Golden Lotus the god Susuram showed that you must complete a great quest to attain the fame and prosperity you desire.

The Even Dirtier, Simpler Monster Conversion Method for Swords & Wizardry to Dungeons & Dragons 5E (playtest update)

Playtest Update #1: I am revising how to do quick and dirty hit points based on my playtest experiences tonight.

As I was mulling over my recent Swords & Wizardry Complete acquisitions it occurred to me that at its core there is a single on-the-fly method to use S&W stat blocks with minimal or...seriously...almost no "real" conversion at all in D&D 5E. In fact most of the conversion work would be in guesstimating DCs for saves and skill/ability checks; the stat blocks themselves can be used with some simple rules in place for quick on-the-fly numeric conversion. The new method below will work with any 1st edition stat block as well, and I think is a bit cleaner than my prior system for conversion.

Here are my conversion method suggestions so far:

Preamble: before converting something in a S&W Complete tome from Frog God make sure they didn't do it already in Fifth Edition Foes; and remember a significant number of the creatures in Monstrosities are already in the 5E MM.

1. Hit Dice
I've mentioned in a prior conversion that you could figure "5E HPs" by looking at the hit dice of the monster and comparing that to the equivalent CR on the table on page 274 in the 5E DMG; there's a simpler formula that gets a good approximation:

The Dirt Simple Hit Point Method: Roll HD as normal (or figure from an average; usually 4.5) and multiply by 4. This will generate a wider range on the hit dice and avoid uniformity. You should still roll regular hit dice for humanoids (like humans and elves) that normally progress with levels, however. PLAYTEST UPDATE: I didn't like the hit points this way in tonight's game, and have revised this to work as follows: use regular hit dice rolls for level/class based humanoids. Take the hit dice, roll or multiply by the average (4.5) and then multiply by 2 otherwise (shortcut: take HD and multiply by 9). X4 is a lot of hit points...do not use unless you want to challenge the party, or have a lot of high damage dealers in the group.

2. Armor Class:
S&W's armor classes include an ascending scale that is very close to what 5E uses....you rarely see S&W ascending ACs that are higher than what you normally see in 5E. If you're using a non-S&W stat block for conversion, or a module for S&W that uses descending AC, remember the formula: subtract descending AC from 20 for 5E AC.

3. Saves: 
On the saves, I think you could give S&W conversions a bit of oomph and also simplify your math by dividing HD by 3 and rounding down; apply this modifier to any saves the creature makes in lieu of determing attributes. If the creature has a noteworthy stat mentioned in its description, apply a +3 or +4 bonus for that stat's saves only.

4. Attack Rolls: 
There are a few ways to do this, neither is any better than the other, but I settled on the following because it is easiest to do on the fly:

The Dirt Simple Attack Bonus Generator: use the HD as the attack bonus. This may mean the monster is 1-2 points higher on average (or lower) than the option one conversion, but it works just fine and is going to give you a range that will be level appropriate or even superior to other monsters without being out of bounds. Bump this up by +2 if the creature is noted for exceptional strength (or dexterity if ranged or finesse) on specific applicable attacks.

5. Initiative Bonus:
In 5E very few monsters have an initiative modifier beyond Dex. My suggestion is divide HD by 3 (round down) and provide as the init mod/Dex mod. If the creature has a noteworthy Dex, use the mod you settle on instead.

6. Damage:
I have worked out three methods, but personally default to the preferred version:

The Preferred Quick and Dirty On the Fly Damage Conversion: add the hit die number to each damage set rolled. So a 5 HD beast with 3 attacks dealing 1D4/1D6/1D6 (average damage 9.5) now deals 1D4+5/1D6+5/1D6+5 (average damage 24.5). If you want a really tough boss monster then add double the HD to each damage roll.

If you compare the above to our HD determination method, to use the example a 5 HD monster has 90 hit points (average) on conversion....going by the DMG chart on page 274 that means it's average damage expression is around 15-20, so we're higher than the average for defense CR right now, which is just fine, I assure you.

Option 1-The Extra Dice Method: treat hit dice as extra damage dice instead. Add those damage dice evenly as multples to the existing attacks. So in the case above with three attacks dealing 1D4/1D6/1D6 from a 5 HD monster, you have 5 extra damage dice to deal: adding it evenly works out as follows: 2D4/3D6/3D6 (for average damage of 24); you can fudge it to make attack expressions line up properly (in this case we assume the D6 attacks are claws so they'd be equal).

Option 2-The Extra Deadly Straight Multiplier Method: if you want the monster to be a terrifying threat, do this. Just multiply the damage rolled by the hit dice of the monster. It's up to you if you multiply any adds, but strictly by the 5E rules you shouldn't. So a 5HD monster with  1D4/1D6/1D6 expression in this gets a damage average of 47.5....yowza!!!! but it does make the monster hit much tougher.

7. Save DCs
Just use this formula for a good average: 8+HD=Save DC. Maybe give it +2 if it has exceptional ability of note in the stat that fits any spell casting ability. If the creature has more than 10 HD then increase the Save DC by 1 for ever 5 additional HD after that (i.e. HD 11-15 = DC 19; HD 16-20 = DC 20, etc.)

8. Converting Save or Die Effects:
For save or die effects, to convert to 5E I suggest the following: figure the damage type (i.e. necrotic or poison), then change it to a save vs. DC against appropriate stat from step 7 above. Then have the attack deal dice of damage of that type equal to the HD or the monster, using D8 (or higher if it seems appropriate). Problem solved. EX: A 7 hit die creature has save vs. poison or die. Convert this into a Save vs. Con DC 15 or take 7D8 poison damage (or 1/2 on successful save). Convert other effects precisely, but maybe add a save after 1/0th of the time has passed for the effect to shake it off. EX: save vs. paralysis or become paralyzed for 1 hour. Allow a save to recover after 6 minutes. Another example: creature does a paralysis for 1 minute, so let the victim save after 1 round.

A little secret: this is going to be a houserule for my upcoming S&W Complete game; I never liked save-or-die even in the 80's, so all save or die effects in my games will deal damage equal to the hit dice instead.

9. Magic Resistance:
You can use the percentage as-is, or just subsitute the 5E standard magic resistance. I do the latter (easier) but either method is fine. Feel free to beef up resistances and immunities on the creature if it looks like it could use some.

10. Converting "hit only by magic weapons:"
In 5E this is always resistance against normal damage, but silvered/magic weapons bypass.

11. Level Drain
Convert that into 5E's "reduced hit point maximum" rule with Save DCs vs. constitution.

12. Movement: 
Divide by 4 and multiply that result by 10 feet for movement.

13. Everything Else:
When you're done, you might see something that makes less sense in 5E without it. For example, MR is a big deal in S&W Complete, but not as much in 5E....a lot of creatures that used to be highly magic resistant now express that through additional resistance/immunity in 5E, so feel free to beef defenses up if needed.

12. Pulling it All Together: The Cheat Sheet
Here's my "preferred list" cheat sheet for conversion. Print it out and stick it inside Monstrosities and Tome of Horrors Complete....congratulation, you've just doubled (or tripled) your monster options for 5E.

THE QUICK AND DIRTY S&W to 5E CONVERSION CHEAT SHEET:

• Size: medium, unless text suggests a larger or smaller creature.
• Hit Dice: Take HD number and multiply by 9. Roll HD as normal (figure a Con mod) for level/class based humanoids.
• Armor Class: use the ascending AC as is.
• Initiative: divide HD by 3 and round down for init mod/Dex mod.
• Save Modifier: use HD as CR to look up save bonus; for quick conversion divide by 3 and round down.
• Attack Bonus: use the hit die as attack bonus (so 5 HD = +5 attack).
• Damage: add HD as a bonus to damage to each damage roll; for boss monsters double the bonus.
• Save DC: 8+HD=save DC. Add +2 if it's a spell casting monster or boss monster.
• Movement: divide by 4 (round up) and then multiply by 10 feet
• Convert Abilities/Save or Die Effects: use the force to guide you; for save or die deal damage of appropriate type equal to HD in damage of monster (with save for half). Convert MR to 5E, and magic weapon immunity to 5E resistance.
• Challenge Rating: calculate it using the 5E DMG or just make it equal to HD

Here's a link to the Cheat Sheet

Done!

Challenge Rating
Now, calculating the CR....this is obviously easy enough to do with the final conversion averages, using the rules on page 274-277 of the DMG, and good to try out since it will give you an idea of what converted S&W monsters look like in 5E under this method. If you are rushed (doing a conversion on the fly) but use the cheat sheet version above, just look up CR by hit die and call it a day.

Sample Conversions:

Malcarna (from Monstrosities, page 313)
Chaotic Evil large fiend
CR 6 (2,300 XP)
Hit Points: 45
Init Mod: +4
Defenses:
Armor Class: 16 (natural plus shield)
Magic Resistance: rolls advantage on saves vs. magic
Resistance: fire, only magic or silvered weapons do regular damage
Immunity: charming, banishment
Saves: +2 on all saves; +6 Charisma
Attacks: 
Multiattack: 1 tail plus 3 weapon attacks
Tail Attack +5 to hit (reach 10 feet, 1 target) 1D8+5 bludgeoning damage
3 long swords +5 to hit (reach 5 feet, 1 target each) 1D8+5 slashing damage each
Movement: 30 feet
Hideous Beauty: targets must make a Wisdom save DC 13 to even attempt to hit a malcarna. Creatures of level 4 (or HD 4) or less roll at disadvantage to this save.
A hideous four-armed snake-tailed demon reminiscent of the Marilith, a half-demon often born of Marilith and mortal relations. 

Spire Monkey (from Monstrosities, page 456)
Unaligned small beast
CR 1/2 (100 XP)
Hit Points: 18
Init Mod: +0
Defenses:
Armor Class: 12
Saves: +0 on all saves; +4 Dexterity
Attacks: 
Multiattack: 3 claw attacks
claws +2 to hit (reach 5 feet, 1 target each) 1D3+2 slashing damage each
Movement: 40 feet (both normal and climbing speed)
Spire Monkeys are two-headed, six-armed little simians that live on rooftops and trees. 

Xole (Monstrosities, page 520)
Chaotic Evil medium elemental 
CR 6 (2,300 XP)
Hit Points: 91
Init Mod: +2
Defenses:
Armor Class: 17
Immunity: fire, cold
Saves: +2 to all
Attacks:
Multiattack: 2 (weapon and tail)
Stone Mace +7 to hit (5 foot reach, 1 target) 1D10+12 bludgeoning damage
Tail Swipe +7 to hit 10 foot reach, 1 target) 2D6+7 bludgeoning damage
Movement: 30 foot (earth-phasing or regular)
Earth-Phasing: xole move through earth in a semi-incorporeal state. It takes 1 movement action to begin the process, then they move normally.
Evil Elementals: Xole are evil and can be affected by protection from evil.
Elemental Mace: the mace can only be lifted/wielded by elementals of stone and earth.
Xole are similar to salamanders, but native to the plane of elemental earth; 7 foot long humanoid torsoes and 10 foot long tails.

Cinder Ghoul (Tome of Horrors Complete S&W version, Page 270)
Chaotic Evil medium undead
CR 5 (1,800 XP)
Hit Points: 54
Init Mod: +2
Defenses:
Armor Class: 15
Immunity: necrotic, fire, psychic, poison, disease, does not breathe
Saves: +2 to all
Attacks:
Slam +6 to hit  foot reach, 1 target) 1D8+6 necrotic damage plus 1D6+6 fire damage plus target must make a Save DC 14 vs. Constitution or it's hit point maximum is reduced by the necrotic damage dealt and can't be regained until after a successful save at the end of a long rest. If the target is reduced to 0 HP on its maximum it dies (and probably comes back as a cinder ghoul).
Movement: 30 feet flying
This ghost-like monster of ashes and charred body parts is a terror to behold.

(Conversion Note: the ToH stat block doesn't mention fire immunity but...come on, it's gotta have it; the rest are implied for 5E undead immunities)

D&D 5E Updates: The Mohrg

Starting now and until I run out of energy I think I’ll take some time do do some conversions of some of my favorite Pathfinder and 3.5 monsters that haven’t yet appeared for D&D 5E….starting with the tentacle-tongued mohrg, terrifying serial killers from beyond the grave….!

Mohrg

CR 5 (1,800 XP)

CE medium undead

Initiative +4

DEFENSE

AC 18 (dexterity plus natural)

HP 91 (14D8+28)

Damage/Condition Immunity: poison, disease, necrotic

OFFENSE

Speed 30 feet

Multiattack the mohrg may attack twice with its slams or once with a slam and once with its tongue

Melee Attack – Slam +7 attack (reach 5 ft, one target) 2D10+5 plus target is grabbed (escape DC 15 strength check)

Melee Attack – Tongue +6 attack (reach 5 ft, one target) gains advantage to attacks on a grabbed target; target must make a DC 15 save vs. paralysis or become paralyzed for 1D4 minutes (target can make a save at the end of each of its turns, ending the effect immediately on a success).

Statistics

STR 21 (+5), DEX 19 (+4), CON 14 (+2), INT 11 (0), WIS 10 (0), CHA 14 (+2)

Languages: common, but many mohrg cannot speak

Senses: darkvision, perception +2 (passive 12)

Skills: Athletics +7, Stealth +6

TRAITS         

Create Spawn: creatures killed by a mohrg rise immediately as zombie with a movement speed of 30 ft (instead of 20 ft typical for most zombies that have had time to rot). When this happens the burst of necrotic energy heals the mohrg for 3 (1D6) hit points per hit die of the slain creature and the mohrg gains the benefit of the haste spell until the end of its next round. On occasion some mohrg generate new mohrg instead of zombies as an effect of slaying a foe. These mohrg are exceptionally dangerous. When a mohrg slays a target, there is a 10% chance that it returns as a new mohrg instead. At the DM’s option any evil character slain by the mohrg has a 50% chance of returning as a new mohrg.

Mohrg are terrifying undead forged from the vile souls of serial killers and murderers. They resemble skeletons with a writhing mass of organs and flesh in their body cavity and skull, from which emerges a hideous tentacle-like purple tongue covered in paralytic slime.

Mohrg are noted for their unrelenting sense of commitment to the evil ways that led them to their fate. When mohrg gather in groups this can quickly lead to a unique and terrifying epidemic of undead in the form of zombies and new mohrg.

Source: D20 SRD and Pathfinder Bestiary, page 208. 

Converting AD&D 1st Edition Monsters (such as from, say, the Creature Compendium) to D&D 5E

New Big Dragon Games' Creature Compendium is an amazing OSR book and one of many (Teratic Tome being a #2 on my list) that are really amazing resources for the modern classic gamer. But some of us run D&D 5E, so must we languish with our meager resources and ignore this OSR goodness? No! The conversion of classic monsters to 5E is actually pretty easy, and the rules for it are buried in code within the Dungeon Master's Guide itself.

The methodology I've worked up to convert AD&D monsters to 5th edition D&D is pretty straight forward, although a bit of handwavium and a steady application of holistic methodology is also involved.

So, here's the quick and dirty conversion guide:

Armor Class: take the creature's AD&D AC and subtract that # from 20. if it's a negative add it.

Hit Dice for Monsters: for monsters of most sorts take the hit die notation and convert it to a CR. Look up the CR on the 5E DMG chart on page 274. Write down the proficiency bonus, attack bonus, the desired hit points within the suggested range, spell save DC and average damage per round. If the Hit Die code includes a bonus then add 1 CR if it's a +1 or +2, and add 2 CRs to the formula if it's +3 or greater, but to the hit point total only (use the base hit die to determine CR for the rest). So, for example: HD 2 is CR 2. HD2+1 is CR 3 for hit points and CR 2 for all else. HD 2+4 is CR 4 for hit points and CR 2 for all else.

Hit Dice for Humanoids: treat the hit dice as actual hit dice, and assuming its a medium creature it has D8s. Figure out the average hit dice in this manner and then cross-reference with the hit point values on the chart to get the base CR and associated stats for the humanoid. This works best for monsters that are otherwise "humanoid" and would have normal hit dice and possibly also class levels. If you're not sure which option makes sense, compare both options for monsters and humanoids and go with the one that feels right.

Number of Attacks: turn this into multi-attack for the # of attacks indicated if more than one.

Damage/Attack: damage totals are much lower in AD&D than in 5E. Look at the damage average figured by CR in the prior step and compare to the damage output. My suggestion is to either tweak the existing damage totals until it averages something in the range of the expected CR, or to simply use a flat # based on the CR average, divided by # of attacks. Remember that the CR damage output is "total" so it would reflect the average damage of all normal attacks in the round.

A Formula for Damage: Figure the average damage out put for all per-round attacks of your monster (that he normally uses per round). Divide the average damage output by CR to get the multiplier; multiply the damage dice to figure actual damage output. Example: a dirt giant does 1D6+2 twice per round, so average damage is 11. It's CR 8 (by HD only) suggests average 5E damage output of 50-56. So divide 53 by 11 and you get 4.8 (round up) for 5. So each fist ought to do 5D6+10 damage now. Oi!

Poison/Energy Drain Conversion: any save or die poison effect should be converted to damage, and can be factored in to boost damage figures above. Any necrotic effect that involves energy drain should be converted into the "hit point maximum" drain that is standard for 5E, although if you really want to go hardcore just leave these as-is, using the CR-based DC for saves vs. poison or energy drain and failure = actual level loss, death, or what-not. Just warn your players in advance so they are ready with backup characters...

Magic Resistance: converting this is really simple (since there's one default type of MR in 5E) but since I don't think 5E's MR captures the full spirit of classic edition MR I suggest instead just going with the MR as defined by the monster (so, use percentiles and the rules in the monster stat block as-is, basically). If you do that, and it's especially tough on PCs compared to standard 5E MR, then consider adding 1-2 CRs for determining the XP value of the creature.

Size: this is a "eyeball it and convert" sort of thing. Most creatures will be small, medium or large but if you think a 25 foot tall monster is huge or gargantuan go ahead and apply that. This can be used to figure out actual 5E hit dice later.

Stats: these aren't too hard to figure out, by figuring out the modifier first and then applying a stat based on that. Alternatively just assume that the creature has a single "Save" stat equal to the CR attack bonus listed on page 274 (so for example a CR 4 creature would have a SAVE state of +5). For more nuanced results use these guidelines:

Strength: If it's clearly a large creature apply a STR modifier as seems fit (and use it to boost damage dealt if you like). If there is an existing bonus to damage for large STR use that to determine the modifier. For example, Fire Walkers in Creature Compendium have a +3 damage modifier +1 due to magic weapon) so they would be STR 14 (+2). Use good judgement....CC for example only provides a +2 STR damage modifier to dirt giant attacks, but that dirt giant in the picture looks more like a +5 modifier to me.

Once you have STR and DEX modifiers figured, you can use these #s plus proficiency or the average attack (whichever is better or more appropriate) for the monster.

Dexterity: if the creature is descriptively fast adjust as necessary, otherwise, assume at least a +1 or better modifier for each of the following: initiative bonus, ambush bonus (either to cause or avoid), any thief skills, any modifiers applied to ranged damage.

Constitution: this is easy, use the HD modifier if any. Add additional bonuses if the creature has poison resistance/immunity or other notable CON-friendly features.

Intelligence: convert directly using the AD&D intelligence chart for monsters (AD&D uses descriptors for INT, but there's a numerical equivalence to draw from). Or just apply a modifier as follows: Animal (-3), Semi (-2), Low (-1), Average (0), Very (+1), Highly (+2), Exeptional (+3), Genius (+4), Super-Genius (+5) (references from Creature Compendium).

Wisdom: set this to at least +2 or better if the creature has clerical or druidic magic. Add +1 for each point of MDA it has, if any.

Charisma: most floaty of all when it comes to monsters add a modifier based on your gut feeling, or at least +1 or more if it appears to have some form of bard-like magic. Set this at +2 or better if the creature has any charm/enchantment effects.

Skills: don't worry about these unless a specific skill-like trait is suggested in the monster description, then set it's rank at a level equal to the determined Stat modifier plus Proficiency bonus (or just use the SAVE modifier if using that simpler rule).

Movement: take the "inches" figure and multiply by 10 to get feet per move action. So a 3" move is 3X10 for 30 feet. This will work for most conversions....mostly.

Spell Conversion: surprisingly few spells in prior editions don't have some analog in 5E, I've found. For those that don't simply crib some notes from spell conversion in the back of the 5E DMG or substitute an appropriate different spell. For Spell save DC, remember to use the CR conversion above to figure the creature's average Spell Save DC modifier, or altenratively figure the spell caster stat (INT, WIS, CHA) from your calculation above and then apply the proficiency modifier by CR.

Converting Monster Special Abilities: converting the myriad special abilities of monsters in AD&D is the really hermeneutic part of the process. Many work fine as-is, while some need some holistic massaging to convert, usually by applying the 5E mechanical application of the effect (such as poison and energy drain which I singled out earlier). On pages 280-281 of the 5E DMG a list of creature abilities can point you to finding the equivalent effect for conversion. Ultimately my suggestion is to convert those traits which have consistent mechanics in 5E, and to use unique traits as close to original as possible in other cases.

Also, remember that judicious use of the advantage/disadvantage mechanic can solve a lot of modifier issues in conversion.

Psionics: 5E doesn't have psionics but some spells like detect thoughts can substitute for psionics if needed. Otherwise do like I did when I actually ran AD&D in the 80's and ignore....

Monster XP Value: The final CR of the creature, one all of the above have been determined, can be figured by applying the logic outlined on page 275 in the 5E DMG. Don't use the CR conversion on hit dice earlier....that's just to get the base stats rolling.

Optional - Figuring Actual Hit Dice: to figure the actual hit die value of your converted creature take the average hit points you determined in the HT to CR conversion at the start, then look on the 5E DMG to figure the correct 5E hit die type by size (if you're not 100% sure of the size just use your judement). Apply the CON modifier you determined (or default to any HD bonus in the stat block), take the average hit die plus CON modifier and divide the HP average by that amount to figure the actual hit dice. Example: A CC dirt giant has HD of 8+6. From this I look up his CR as 10 and get a HP range of 206-220, and I can set his CON as 22 (+6). So in 5E he's a large (11 foot tall) monster with D10 hit dice (5.5 average per roll). So 213 average hit points divided by 5.5 plus 6 for con for a total of 11.5 (round up on this). That works out to 19D10+114 for total hit dice! Which is normal for 5E, I assure you.

So with all of that, here's a sample conversion from the Creature Collection:

Dirt Giant (CC, page 30)
Large Giant TN
AC: 18
Hit Dice: (218) 19D10+114
Proficiency Base: +3
Move: 30 feet
STR 20 (+5), DEX 10 (0), CON 22 (+6), INT 9 (-1), WIS 10 (0), CHA 8 (-1)
Optional Save Stat: +6
Senses: blindsight; otherwise naturally blind. Perception +3 (passive 13)
Attacks:
Multiattack: two strikes per round with fists
Melee attack - hand gouges +8 attack (10 feet one target); (27) 5D6+10 bludgeoning damage.
Traits: 
Blindsense: Dirt giants are blind and detect movement through blindsight (see 5E MM) using a clicking sonar effect (think giant burrowing cordyceps zombies from The Last of Us). Silence spells can eliminate their blindsense creating an "invisibility effect".
CR: 10 (5,900 XP)

The dirt giant was an easy conversion. The next one is an example of holistic conversion in action:

Kam Warrior (CC, page 38)
Medium Humanoid TN
AC: 15
Hit Dice: (26) 4D8+8, then (13) 2D8+4 twice, then (7) 1D8+2 four times (total of (80) 12D8+24)
Proficiency Base: +2
Move: 30 feet
STR 14 (+1), DEX 11 (0), CON 14 (+2), INT 10 (0), WIS 10 (0), CHA 10 (0)
Optional Save Stat: +3
Senses: normal; Perception +2 (passive 12)
Attacks: 
Melee Attack - long sword +3 attack (one target, five feet); (5) 1D8+1 slashing damage
Traits:
Cloning: when a Kam warrior of 4 hit dice is reduced to zero hit points two kam warriors manifest on its next round as a bonus action as close as possible to where the kam warrior fell the prior round. Each of the new Kam warriors have the same stats but 2 hit dice. Each of these warriors, when reaching zero hit dice will split on the next round in the same manner into two 1 HD kam warriors. 1 HD kam warriors will die when reaching zero hit points. Each reductive iteration of the kam warrior has the same base stats and normal equipment of the original; only hit dice change. Magic gear does not replicate and stays with one clone at a time.
CR: 2 (450 XP) (I bumped it one because the damage output goes up each time you hack a Kam warrior down)

So....hopefully these guidelines, when applied to the monster design rules from the 5E DMG, will dramatically expand your choice of freaky denizens for delving. I strongly encourage you to pick up the Creature Compendium as well as the Teratic Tome as two great places to start with your OSRification of 5E!