I posted briefly about this on G+. I was doing some google searching that somehow managed to lead to "VENOM" from Spiderman. I think I'm going to throw a demonic symbiote at one of my players. Here are the stats thus far.
When the demon attempts to take over the PC's body, have them roll a saving throw vs spell at -2 on the roll. (Cuz you want this to happen to them!).
Once possessed.... (k not the right word). Once the 2 are melded the following occurs:
The Demonic suit acts as Plate Mail, as far as AC is concerned.
The PC becomes immune to charm spells, however susceptible to Sleep spells (Saving throw at -2 on roll).
Have the player roll 3D6 and mark this down as "Self" stat.
Whenever the player decides to do something "good" orientated there is an internal struggle. (this doesn't need to happen every-time). In this instance i might decide behind the screen to use +Venger Satanis 33% mechanic.
Roll in front of the screen a contest of wills between the PC and the Demon. roll a 20. Whoever is highest wins. In the case of the Demon, the PC will not do what they intended but the complete opposite.
Deduct one point from the "Self" stat. Whenever that stat reaches 0, the PC has fully become the demon and has lost all self.
You may consider adding one point back when the PC wins that is up to you.
In order to remove the symbiote, the following must happen.
1. a Remove curse spell
2. Kill the PC, to detach the symbiote
3. A Resurrection spell