The troll and elven lineage is the idea that night elves descended from trolls. Whether this is the case, is unknown since Blizzard's history of Azeroth does not make such a claim. It does state that the night elves are descended from "a primitive tribe of nocturnal humanoids" who were feral and nomadic (a few sources call this tribe the "Kaldorei".) This is a point of some debate amongst fans. That the race in question was troll or some ancestor of the troll is certainly one interpretation and would seem to be borne out in light of the physical similarities between night elves and trolls, but there is no clear statement of this as fact.
The original information to support the claim can be found in the in-game text "The Twin Empires", which states:
- "...ancient texts speak of a small faction of trolls that broke off from the Amani Empire and founded their own colony in the heart of the dark continent.
- There, these brave pioneers discovered the cosmic Well of Eternity which transformed them into beings of immense power. Some legends suggest that these adventurous trolls were the first Night Elves, though this theory has never been proven."
Yet even as seen above even the in-game text states that there is no proof for the 'theory'.
Alternatively other legends imply that Night Elves descended from the offspring of Elune, she herself appearing to be Night Elf in form. By this legend, it would explain how Night Elves share such similar physical appearance with Dryads, and Keepers of the Grove.
According to an article on World of Warcraft's website which discusses the issue, there is no evidence that proves either explanation, and truth is unknown. Much the same is said in in-game books and in the RPG as well.
Other possible races
It is stated in LoC that primitive humans then known as Azotha were nomadic and nearly as primitive and barbaric as the trolls. They would also be a good candidate for ancestor of the night elves. However humans are descended from Vrykul and this leaves little place for speculation that they may have further evolved into Night Elves.