The Troll is a massive beast that made a short appearance as a minor antagonist in Harry Potter and the Philosopher's Stone.
This troll's race named mountain trolls (Troglodytarum alpinum), who being largest, weight over a ton, and the most vicious kind of their species. They are often depicted as bald and pale-grey. They sometimes use Graphorns as mounts, which results in scars due to the Graphorns not liking it.
Tricked by Quirrell and Defeat
The Troll was lured into Hogwarts by Professor Quirrell who claimed that the Troll was in the dungeons, but this was just a lie as the troll now already roaming in the girl's bathroom, where he found and attempted to kill Hermione. Harry Potter and Ron Weasley came to the rescue and succeeded to beat the troll unconscious, though not without various difficulties: The troll completely ignore them due to its stupidity and too focus in attempt murder on Hermione.
This prompt Harry to jumped on its back and having his wand stuck in one of his nostrils. This, of course, caused the troll snapped and angrier than ever. He grabbed Harry by his mercy and tries to smack the boy with his club. Like the previous attempts to kill Hermione, the troll was distracted so much that he unaware that Ron uses levitation spell to take his club away with Hermione's help. When the troll realized it, it was too late as Ron led the club to fall onto his head, instantly knocked him into submission.
Motivation Behind Troll Incident Revealed
What happened to the troll after this incident is unknown, though it is likely the troll was killed off or it had escaped. Whichever way, the Troll is never seen again. Except that it is hinted in the later books that the troll was returned to the mountains. Later Quirrell reveals to Harry that he was the one who released the troll and why he did it.