Clarent

In the Alliterative Morte Arthure, a magnificent sword kept by Arthur in a vault at Wallingford. This was a sword only to be used for ceremonies, such as knightings.

Other than Arthur, only Guenevere knew of its location; therefore, when Arthur saw Mordred wielding it in battle, he knew that Guenevere had betrayed him. Mordred carried it throughout the war with Arthur, and perhaps killed Arthur with it.