In the writings of Middle Earth, Trolls first appeared at the Fifth Battle of the first age (Nirnaeth Arnoediad). According to The Silmarillion in that battle a man named Húrin stood alone against the troll guard of Gothmog, the Balrog captain of Morgoth's army. He single handedly killed seventy trolls before orcs managed to capture him through sheer force of numbers (Hurin's brother was a distant ancestor of Aragorn).
The origin of trolls is not known, though Treebeard the Ent, told Merry and Pippin that trolls were made by Morgoth in mockery of the Ents, which may account for the large size of trolls. In earlier years, trolls were little more than large beast with no language. In later ages, Sauron began to train trolls and teach them basic language skills. This is evident in the Hobbit where Bilbo and his dwarven companions were captured by trolls, who were able to speak with them. They still weren't too bright though. According to Aragorn, trolls don't build, but instead live in more rudimentary shelters like caves. Another thing also learned from the Hobbit, is that trolls turn to the stone they were made from if they are exposed to sunlight, which restricts them to night-time activities. This was the fate of the trolls Bilbo was captured by.
Types of trolls
The only mention of cave trolls was during the fellowship's journey through Moria, though Gandalf's recognition indicates that they were a variety familiar to him and possibly found elsewhere. They were described as having a green scaly hide and flat feet with no toes.
The cave troll depicted in the Peter Jackson film differed from the ones in the book, not only in appearance, but also in nature. A single cave troll was extremely difficult for the entire fellowship, including Gandalf, to defeat, which almost implies that a troll is more powerful and feasome than the Balrog which Gandalf alone defeated (it certainly got a lot more screen time). Seventy trolls being killed in the first age by a human makes it even more remarkable that a single troll was portrayed as being so formidable.
Some games based on Lord of the Rings such as FASA's MERP (Middle Earth Role Playing) have included half-trolls in their lists of species. The only indication of half-trolls in Tolkien's writings was at the battle of the Pelenor Fields, where there was a brief mention of men from far Harad, black men like half-trolls with white eyes and red tongues.
These trolls were common in hilly regions such as the areas to the north and west of Rivendell. Aragorn's grandfather Arador was killed by hill trolls in year 2930 of the third age. It was in this region (marked on maps as the trollshaws) that Bilbo encountered trolls.
Trolls from the mountain regions were common in the service of Mordor. It was this kind of troll that swung the great battering ram of Mordor at the gates of Minas Tirith.
In the Third age, a new breed of trolls began appearing. These Olog-Hai were larger and more powerful than normal trolls. They were most likely created by Sauron. Olog-hai were cunning and could endure the sun as long as the power of their master continued. They spoke only the black speech of Mordor.
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