Quirinus Quirrell's first mountain troll

"Troll in the dungeons! Troll in the dungeons! Thought you ought to know. (passes out)"

- Quirinus Quirrell, distracting the Great Hall

Quirinus Quirrell's first mountain troll was a male mountain troll brought into Hogwarts School of Witchcraft and Wizardry on Halloween of 1991 by Quirinus Quirrell, possessed by Tom Marvolo Riddle, in an attempt to steal the Philosopher's Stone of Nicolas Flamel.

Charms
At the beginning of the holiday, the first year Charms class were to practice the Hovering Charm. Professor Filius Flitwick told them to use the incantation Wingardium Leviosa.

Later, that day, Ronald Weasley hurt the feelings of Hermione Granger. She was later seen by Parvati Patil, crying.

The Feast
Trying to cover his guilt, Ron just ate. Suddenly, Professor Quirrell distracted the Great Hall by "warning" them of the troll he had let in.

"Silence! Everyone will, please, not panic!"

- Albus Dumbledore

Headmaster Albus Dumbledore told everyone to head back to their dormitories, when Harry Potter remembered Hermione.

The Troll
They ran toward the bathroom and spotted Severus Snape, following Quirrell to the third floor corridor. They, unknowingly, locked the troll in with Hermione. Realizing their mistake, they ran in and Ron screamed at it and threw something at it. Harry, meanwhile, jumped on the troll and shoved his wand up its nose. The troll grabbed and Ron hovered the troll's club over its head and knocked it out. This was the start of Harry and Ron's friendship with Hermione. It is unknown what happened to the troll.

Aftermath
Harry, Ron, and Hermione originally assumed that Snape was behind the "troll buisness", but Harry later found out that it was Quirrell/Voldemort, not Snape.

Quirinus Quirrell
"I do say I have a gift with trolls."

- Quirinus Quirrell

Quirinus Quirrell was able to control this troll, a rare talent. He said later that he had a gift with trolls. He even used a troll to help protect the Philosopher's Stone.