4 Degrees of Possession:
1st Degree - Mindshadow Demon in question only appears as an evil thought in the subject's mind. Only those on the edge are likely to be affected. Simple interventions can clear this, if the subject is aware of it.
2nd Degree - Power Struggle Demon in question is a major force in the target's mind. Although the subject still has control, this is fragile and could be comprimised by pre-existing mental instability. It may take some effort, but moderate rituals can clear this.
3rd Degree - Possessed Classical possession - Demon is in full control of the victim. Usually requires full exorcism to clear.
4th Degree - Consumed The victims physical form is used to build a physical body for the subject demon. This transformation is usually very horrifying depending on the nature of the demon. It may appear as a simple morphing, like lycanthropes, or more spectacular, including the demon form bursting out of the victims body.
This form of possession should be dealt mechanic-wise as a powerful death spell.