One of the four Swords of Power, this sword was wielded by Mordred, then used by Cercunnos (The Horned God) to kill King Arthur (and Mordred too!)
Later, Nicholas Flamel found the sword. He kept it safe for centuries until he gave it to Josh Newman. When Cercunnos and Dee attacked Palamedes's castle of car parts, Josh left Clarent behind when escaping and Dee found it.
Dee then merged it together with Excalibur to create one sword. (He did not know they could merge before he merged them.) When Dee fell into a leygate in the Tower of London that led into an unfinished shadowrealm, he found out about the true power of the swords: making leygates. This sword eventually became part of the Hook of Aether.