I can replace the demon summoning stuff with another power that thematically fits his character, but I don't really want to leave him with just the metamorphosis. It's not something that I want him to use often. (He doesn't just get powers when he transforms, he literally lets a malignant entity take the wheel.) So I'm alright with scrapping the demon summoning but I want him to have something else to use when he's not using his trump card.
There was also the "sacrifice" aspect of the power I really liked. I wanted Luke to give more and more of himself in order to save others.
I'd like to think of a power that is both demonic in nature and requires sacrifice, but I really can't think of anything else.
If it's an issue of my character not being mentally scarred enough, I can fix that instead.