Excalibur

Excalibur was the sword of King Arthur of England. When he fell in battle, Merlin placed it with his body in a dimension-crossing spaceship and hid it at the bottom of Lake Vortigern. In the late 20th century, the Doctor and Ace discovered Arthur's spaceship and retrieved Excalibur. (DW: Battlefield)

The Master once tried to steal it back from the Doctor. (VG: Destiny of the Doctors)

Captain John Hart jokingly suggested Excalibur as a name for Torchwood Cardiff. (TW: Kiss Kiss, Bang Bang)