There are many diffrent answers, it depends on what story it is, but the most popular answer is that they turn to stone forever.
They turn to stone when the are exposed in the sun
On his first attempt at burgling in "The Hobbit," Bilbo is caught by the trolls and nearly eaten before Gandalf intervenes and saves him by tricking the trolls into staying out until dawn when they turn to stone. This encounter marks the beginning of Bilbo's adventures and his growth as a character.
The Dwarves will get captured by the giant spiders who plan to eat them, Bilbo rescues them, only for them to be captured again by the Elves.
They continue on their journey towards the Lonely Mountain.
They turn into little trolls that make your life miserable.
All three trolls are turned to stone by the first light of the sun.
Bilbo and the dwarves are caught first by trolls, then by goblins. they are trapped up in a tree because of goblins and Wargs
they are amber trolls
After they caught Bilbo, the dwarves came looking for him. As they got around the fire the trolls popped a sack over their heads. It all started when they caught Bilbo. As they were questioning him, Balin came in, quite unsuspecting of anything. The trolls jumped him before he knew what's what. Then they hid behind the trees out of the light and waited for the other dwarves, who they promptly caught in a sack.
No you idiot. Trolls are trolls. Demons are demons. The clue's in the name, baby.
There is no specific collective noun for trolls, in which case a noun suitable for the context of the sentence is used, for example a band of trolls, a trouble of trolls, or a tribe of trolls, etc.A more recently coined collective noun is an irritationof trolls, a reference to internet trolls. At least it's the only version that's clean enough for this site. I myself would use a cowardice of internet trolls or perhaps a pusillanimity of internet trolls.
the trolls capture all the dwarves but not Bilbo b/c he his smaller so he hides then the trolls argue about what to do w/ the dwarves and how to eat them then gandalf (who had left them a while ago) comes back and imitates the troll to keep them arguing until morning and they get turned to stone