Cast-offs

A being of incredible divine power regularly creates  or borrows bodies. It imbues them with tremendous power, does its work, then leaves.

Someone in the party has amnesia, total or for a period of a few months or years. They also have some sort of divine powers. A spellcasting or magical class of some kind works best for this. They regain bits and pieces of memory, showing them periods where they were under the control of the divine being. Class abilities or bonus powers are derived from the energy wielded by that being (fast healing, damage resistance, whatever).