Kas was Vecna’s friend and Lieutenant. He assisted Vecna in taking over Oerth. After a short period of working together. Vecna gifted him with an immortal Vampire body and a sentient sword, called the Sword of Kas.
Eventually the voices in the sword convinced Kas to betray Vecna and a legendary battle ensued where Vecna lost his left hand and eye and Kas lost his sword. Vecna died in the legendary battle and Kas was shunted into a rift never to be seen again.