Possesion:

A ‘disease’ that could have been unleashed by Scions and their meddling with bringing demons into the world, or possibly high level mobs could do something like this to you for various reasons.

How it would work:

The act of being possessed would give the recipient bonuses based on what demons inhabited his body. Examples:
-Muckalucka = +5 Str, +5 Con, +10 dam.
-Muchalucha = +5 Int, +5 Wis, Boost in casting level by 2.
-Himahoka = +5 dex, Reduction in size, boost to dodging.

Then, depending on whom your possessed by, the demons would try to “talk” you into doing things.

A voice in your head says, “Kill Woru, his presence offends you.” (Muckalucka)
A voice in your head says, “The black rod you carry is for the weak, sacrifice it to me now.” (Muchalucha)
A voice in your head says, “Kill something now!!” (Himahoka)

If the player didn’t follow the advice of his demon, the demon would become angry and attempt to possess him, making him auto attack things at random, auto-chase people who you’re fighting, shout obscenities, and possibly teleport around randomly.

The possession could be removed by mob or player healers. For evil characters, it may often be worth keeping the possession, as it adds strength at the price of having to do some evil things you’d probably do anyway. Goods would have to lose the possession as quickly as possible as they couldn’t go around killing other good people to appease the demons will.

...Rome...

I could see something like this where a PC opens themselves to possession then gains the power of the demon, but has to keep making WIS, INT saves to keep it under control.

But as for involuntary, nah.