Cusack is one of the last villains on the show, a demon of extremely high rank. I thought he was a little bit lame.
Him and his counterpart Chandler were originally a being known as the Original Demon. The Demon King punished him for rebelling and split his body and soul into two beings. Cusack was tasked with training Zeldris.
He’s very loyal to Zeldris, I have to respect him for that.
He’s the one who technically kills King Arthur, by using a spell to enter his mind and make Arthur plunge a sword into himself.
He actually tried doing the same thing to Merlin. And when he tries to attack her on the mental battlefield, he embarrassingly got his butt whooped.
Not long after that he joins bodies with Chandler to become the Original Demon again. While they ultimately lose, he was one of the few characters to actually defeat Escanor. But with Darkness dwarfing his strength, that was kind of cheating.
He survives that battle and reverts back to his Cusack form. He finds an injured Zeldris and realizes he can make him as the next Demon King. Cusack murders Chandler and tries to put all the Commandments into Zeldris in a secret cave.
Zeldris was too weak to tell him that that would just make his body a vessel for his terrible father. And when the Demon King takes over Zeldris’ body, he easily kills Cusack. Good riddance.