There was a post on troll origins in people's campaigns over at rpg.net and while answering it I thought, "I should add this bit to my blog." And lo, so it was!
Long ago, about ten thousand years or more in the distant past, during the fall of the Prehunat Empire, the trolls were a sylvan race not unlike elves who at the dawn of time were an innocent people. As the fable goes, the first of their kind was a terminally ill woman who would travel to a reflecting pool from which it was said those who slept by the water would experience strange healing dreams. There she shunned her sickly image in the water but made a comfortable bed in the grass beside the pool, and she did indeed dream. A voice spoke to her, ageless and ancient, with a strange quality that suggested that the originator of the voice was, itself, dreaming, perhaps even of her. In the dream it offered her perfection and immortality, if she but honored the sleeping entity. It revealed its name as Tsathoqqua, and said that like the frogs of the earth who bore down into the mud so too had it, long ago, and it had ever since been trapped below.*
The woman considered the voice's offer and eventually assented to its wishes. In doing so, she was changed into the first troll, as her flesh grew rubbery and pallid, but her immortality through regeneration (and a racial memory) was made manifest. Her form became hideous, and she cried out against the change, but Tsathoquaa explained that she was the purest form of beauty he could imagine. This first troll woman was known as Sakarta (and indeed all endreti trolls afterwards would name their first queen after her). She grew mad from her change, and at first lashed out with her newfound beastial rage against her fey kin. In time she grew lonely, and in a pact with Tsathoquaa she would capture her kin in the night, take them to the pool, and subject them to the dark god's change. Thus were the first of the endreti trolls born.
*Tsathoquaa in the Warlords era of Lingusia is revealed to be one of the twleve chaos titans called the Skaeddrath, and the pool from which the first trolls came was also the birthpoint of many horrible monsters over the eons. It is located deep in the eastern mountains of the Amechian region (the Sea of Amech region in the Warlords era).