Only then can the right potion be discovered to exorcize the ghost of Romney for good.
c.1400, "to invoke spirits," from Old French exorciser (14c.), from Late Latin exorcizare, from Greek exorkizein "banish an evil spirit; bind by oath" (see exorcism).
Sense of "calling up evil spirits to drive them out" became dominant 16c. A rare case where -ise trumps -ize on both sides of the Atlantic, perhaps by influence of exercise. Related: Exorcised; exorcising.