Clarent was a sword of peace as opposed to war in King Arthur’s court at Camelot. It was a sword used for ceremonies such as knightings. It was the sword, and not Excalibur that King Arthur would lay on the Round Table.
The sword was stolen by Modred, the son of King Arthur and his sister Morgause. Modred raised an army to rebel against the legendary King. When King Arthur faced Modred in combat Modred fatally wounded King Arthur with Clarent. King Arthur also managed to slay Modred with Excalibur.