If a psyker rolles a 95-99 on the perils of the warp test, he stands a chance of becoming possed by a Daemon and becoming a Daemon host. If the party expels the demon without the possesed character dying, he lives but takes a terrible penalty. Currently if the character survives the possesion he gains 4d10 corruption points, which sort of makes sense from having a demon go cavorting about in your body.From what I've read here, it sounds as if the current rule is a could be jazzed up a bit. The article would indicate a contray event, where the act of expelling a demon would make you incredibly resistant to the effects of chaos and its tempations. There is even a space marine chapter that uses that effect to create uncorruptable soldiers. Possesion in this way could even lead to a rather enjoyable roleplaying experience, similar to the Nascent Psyker advancement, where the players must try to decide what to do with this new potential member of the Illuminati.

Mechanically, I'd recomend a new kind of Elite Advancement to describe the event.

Restrictions: None

Cost: (300XP or maybe more?)

This advancement is only be available to players who have suffered Daemonhost possesion and had the demon expelled. This could be caused either by the demon being killed and the player surviving (possibly by burning a fate point), purged by a Sister of Battle, or other means. This can only be taken with GM permission, as this Elite Advancement can alter the tone of the game in serious ways.

Effects:

New Talent: Entropy Get it, cause entropy enforces stability on chaos. This Talent allows you to remove corruption points in a way identical to insanity points is this enough, or too much?

Gain the Adepta Sororitas Talent Pure Faith Wanted to reflect the resitance to Chaos in a way that would not be too overpowered, so I used current in game rules

Gain the Talent Resitance (Corruption) Just a +10 on rolls to resist the effects of Chaos

From now on the Illuminati may seek to find and recruit the character. The Ordos Malleaus will also try to find and possibly kill the character. What side of the fight the your Inquisitor is on will be up to the GM, but it could lead to some very interesting situations in game. Additional Elite Advances such as Peer (Illuminati) could be provided later at the GMs discretion, depending on how their own particular game unfolds.

And that's the pitch, I'd love some feedback and maybe this could be worked into a good and balanced house rule.

If you want to give someone a talent like that, I'd suggest something over 1k experience. AND/OR using the straight up Untouchable talent found in DDG. Instead of using it as a character creation talent, allow it to be bought in this rare circumstance as one of those 'other' forms of the talent described in the book.

According to some of the novels, if a Psyker is possessed and the Daemon is expelled then the Psyker becomes one of the Illuminated. Basically this means they are immune from further Chaos taint. Game wise I'd say they can not be possessed or suffer mutations unless they actively turn to Chaos or agree to be possessed or mutated.

I would not make them Untouchables as they are anti-psykers. They are blanks that can not be directly effected by psionics nor can they use any kind of Psi power.

After re-reading your suggestion I'd modify it slightly. Cut any corruptions points gained in half, or even eliminate them, as long as you as the GM decides they are not acitvely seeking out corruption. If they knowing open the demon lore book knowing what it is give them the full points. If they do something but have good motives cut or eliminate the corruption points. This could replace your Entropy talent.

Also as I said earlier I'd make them not possesable unless they agree to it. I am curious as to why the Inquisition would try to kill the Illuminated character however. As they can not be possessed I'd say they would be the perfect recruit for Malleus.