He and Rothbart conquered the Forbidden Arts together, but Rothbart took all the power for himself and drove Clavius underground.
Living in Castle Mountain, Clavius planned to kill Prince Derek, the one who killed Rothbart and took his castle at Swan Lake, so he could enter his castle and get the Forbidden Arts, which was a crystal ball containing magical powers - the power to create, change, and destroy. His servant Knuckles set some traps for Derek, but he overcame them all.
So Clavius kidnapped Derek's mother, Queen Uberta, and held her hostage so Derek would come to him. Knuckles set more traps for Derek, from which he was rescued by Odette, who had Bridget turn her into a swan. Clavius was able to get the Forbidden Arts and absorb its power. Derek fought Clavius, who gained the advantage by his magical powers, but Jean-Bob jumped on him to distract him at the last second, and Clavius killed him (from which he was revived by moonlight on the lake at the end of the film).
Then Derek grabbed the orb, which would explode if it fell to the ground or if blasted by the power to destroy. As he and the others made their escape in Clavius's hot air balloon, Clavius jumped onto it and tried to attack Derek and take the orb back, but Derek knocked him over the edge onto the dock, accidentally sending the orb falling as well. When the orb hit the ground, it exploded and caused the mountain - really a dormant volcano - to erupt. Everyone was able to escape except Clavius, who had no means of escape and was consumed by the molten magma and magic explosion.