Effects of being possessed by a ghost?

How would one be affected if one was possessed by a ghost?

depends on if it’s an actual ghost (entity who doesnt want to move on) or residual energy caught in a loop, if the former it’s no different from being possessed by a demon, angel, or whatever else you allow into you. If the latter you take on possible traits of that residual energy as in if the energy is angry/toxic you while that energy is attached to you can take on those traits.