Caliburn

The Sword Caliburn is one of the eight talismans of Sir Cabirus, representing the virtue of Justice. Molded after a sword from 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, probably by the wizard Tyball. The blade was hidden in a secret room in the third level, while the hilt was placed on the tomb of Sir Cabirus on the fifth level. The Avatar found them there in Ultima Underworld I and used the services of Shak the blacksmith, to forge them back together. It was cast into the lava of the Abyss in the chamber of virtue later, to banish the 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.