Under the item description of the Staff of the Forgotten One in the ToA Hardcover, we see that a spirit inside the staff might try to possess the wielder:
The Protection from Evil and Good spell can protect against some kinds of possession:
Until the spell ends, one willing creature you touch is protected against certain types of creatures: aberrations, celestials, elementals, fey, fiends, and undead. […] The target also can’t be charmed, frightened, or possessed by them. […]
If I were under the effect of Protection from Evil and Good, am I immune from getting possessed by the ghost of the staff?