Binder

Binders are another class from Tome of Magic along with the truenamer and the shadowcaster. They contact beings from beyond the known universe (known as vestiges) to inhabit their body for a while and experience the real world again in exhange for special powers. There's a binding check involved that determines whether the vestige can influence your actions. If they win, you have to act in a certain way in certain situations or get a stacking -1 penalty on pretty much everything that only goes away when the vestige leaves. If you win, they just sit nicely along for the ride. Religions are generally against binders because their gods don't really like it when a mortal chump can just doodle a seal on the ground and contact a vestige while the gods can't do anything about vestiges.