Var the Stick — Piers Anthony

Sol controlled the mountain and Sos the Empire. Two worlds so completely different from each other that they could not exist together on the same planet. There had to be war...

Var was the chosen one. Half man, half animal, a mutant victim of the blast, he would have continued to live as a savage if Sos had not rescued him from the badlands. And now Var was called upon to repay that debt, to risk his life as the champion of the Empire in a duel he was secretly afraid of winning...