Caliburn

The sword Caliburn is one of the eight talismans of Sir Cabirus, representing the virtue of Justice. It appears in Ultima Underworld I

Description
Molded after a sword from Earth, this sword is said to cleave Truth from Falsehood. It is said 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 on the third level, while the hilt was placed in the tomb of Sir Cabirus on the fifth level. The Avatar found them there 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

 * This sword really can't be damaged, and is one of the best weapons in the entire game.
 * Name of the sword is a poke on the sword appearing in the legend of King Arthur, Excalibur.