Sounds nice idea, so which fiend you thought would play such role? They are +1 SR and I recall they should have quite high charisma score (or able go ethereal, as succubus is marked as classic example of one).<br/><br/>Fiendish Codex: Hordes of Abyss introduces some spectral demons (obyriths) so they could be able possess monsters (and one is even 'haunting' sort as it has telekinesis at-will ability).