Caliburn

The Sword Caliburn is one of the eight talismans of Sir Cabirus, representing the virtue of Justice. Molded after a sword form Earth, this sword is said to cleave Truth from Falsehood. Fact is, that the sword can't be broken in combat. It is a strong symbol of virtue.

Caliburn was broken in two, when the groups fought for the talismans. The blade was hidden in a secret room in the thirs level, while the hilt was put on a tomb on the fifth level. The Avatar found them there in Ultima Underworld I, and used the services of Shak to forge them back together. It was cast into the lava of the Abyss later, to banish to Slasher of Veils.

Trivia

 * It is really true, the sword can't be damaged, and is one of the best weapons in the entire game.