Sell your soul?
Not on purpose. In fact, even his worship of the elder evil is more 'religious rite to appease and keep imprisoned' than 'muahahaha i wanna destroy the world'.
I'm thinking bonus feats from taint. Heroes of Horror.
He could certainly be tainted. I'll have to look those rules up. He's certainly not evil, though, if it's an issue.
Work for elder evils for 5.
I got that one, yup.
Sell your soul for 2.
Is this in Heroes of Horror, or what? He could technically have had his soul sold, since he's a created critter, and was implanted in this particular body, in a way.
Does it have to be willing?
Get all tainted up for good times that daddy likes.
Yeah, I'll check it out.
Thanks for the ideas, everyone!