Schizophrenia/Possession
A Pureblood Wizard is diagnosed with a case of schizophrenia. His friend/lover found out that he was actually being possessed by irate dead relatives.
The friend/relative had to find out the names of each possessing ancestor in order to force them to leave the victim. The case was complicated because the victim was hit by a Dark Curse before he started exhibiting schizophrenic signs.