The Trolls' lair was a troll-hole that was made by three trolls - Tom, Bert and William - who captured Bilbo and the dwarves as described in The Hobbit. Made from a cave, the lair served as the three trolls hideout from the Sun, their treasury and storeroom.
Description
The cave floor had bones and an untidy litter of plunder, from brass button to pots of gold. On shelves was a good deal of food, along with a barrel of ale. Clothing of victims hung on the walls as well as several swords. There was also a horrible smell in the air.
After Gandalf had rescued Thorin and his party from the trolls during the Quest for Erebor in T.A. 2941, he reminded them that the trolls must have a cave or hole to hide in from the sun. Searching about, they followed the marks of stony boots uphill to a big stone door hidden by bushes. Pushing and incantations by Gandalf did not open it. Then Bilbo asked if the key he had found would help. It did.
The party took much of the food, the barrel of ale, and the gold. Gandalf and Thorin took two swords with beautiful scabbards and jewelled hilts, and Bilbo took a knife in a leather sheath (which for a hobbit was as good as a short sword). Outside the cave the party ate and rested, and then buried the pots of gold near the river with many spells put over them.
Later, in Rivendell, Elrond read the runes on the swords of Gandalf and Thorin. He revealed that they were very old and had been made by the High Elves of the West, in Gondolin. Thus from the Trolls' lair Thorin obtained Orcrist (meaning Goblin-cleaver in Sindarin) and Gandalf got Glamdring (meaning Foe-hammer in Sindarin). Much later Bilbo would name his short sword from the Trolls' lair "Sting".
During the War of the Ring on October 18 T.A. 3018 Frodo Baggins, Samwise Gamgee, Meriadoc Brandybuck Peregrin Took and Aragorn passed by the Trolls' lair by accident and saw the three trolls that had been turned to stone outside.