Since Wyll turns demon, switch his race from human to something something demon (no idea which type) and give him some appropiate abilities.
You mean a Devil, Mizora would never turn Wyll into a Demon, Devils and Demons were fighting eachother during the Blood War,
As for Devil Wyll he gets Fire Resistance, Cold Resistance, Lightning Resistance, Poison Resistance, Slashing Resistance, Piercing Resistance, Bludgeoning Resistance and Dark Vision, pretty much all the stuff Cambions like Zhalk have.
