Inner spirit

Energumen - a person possessed as by an evil ghost, spirit, or entity.

Your character is a hunter of energumen. It's a typical fantasy element, sometimes placed in current times, but to be different, set it in the future. On Earth? On Mars? On a starship? On some other world of non-humans?

Does the energumen need killed or is the goal to remove the evil spirit? (Then what?)

Are the energumen obvious? Does the person know they're possessed? Does the person act in concert with the evil entity? If obvious, are they seen as "crazy" and how do you distinguish regular crazy people from possessed crazy people?

Is the entity truly evil? Or does just it need a host to survive in a symbiotic relationship, and the two can coexist together? Perhaps it's only the non-possessed who find the idea disturbing and the energumen is fine playing host. But if a person doesn't accepted the spirit until the spirit joins with them, then is it free choice?