What sword does Bilbo find in the trolls' cave?

Bilbo finds the sword Sting in the trolls' cave. After Gandalf causes the three trolls to argue until dawn, sunlight turns them to stone, and the company searches their hidden hoard for useful weapons and supplies.

Sting is an Elvish blade, originally made in Gondolin during the First Age. Although it is technically a dagger for an Elf, it is large enough to serve as a sword for Bilbo, who is much smaller than a human or Elf. The blade has a distinctive magical property: it glows blue when Orcs or Goblins are nearby.

Bilbo later names the weapon Sting after using it against giant spiders in Mirkwood. The name reflects the blade's small size and his success in defending himself and the dwarves. It becomes one of Bilbo's defining possessions and is eventually passed to Frodo Baggins, who carries it during the quest to destroy the One Ring.

The other swords found in the trolls' hoard belong to Thorin and Gandalf. Thorin takes Orcrist, while Gandalf claims Glamdring, so those weapons are common alternatives but not Bilbo's discovery.