CR/APL after giving a non-Summoner a fused eidolon?

[Malaqai]

How much would a non-Summoner's CR increase ift he character were to acquire a fused eidolon (a la synthethist) that used the character's class levels to determine the eidolon's strength?

Hello all. I am going to be starting up my first campaign (Pathfinder homebrew setting) in a couple weeks. One of the factions in the setting is a new cult heralding the Return of the gods to the world. To prove it, they have begun conferring blessings that imbue the receiver with divine strength. In truth, the blessed are opening themselves to demonic possession who are using the hosts to gain access to the Material Plane. These demons allow their hosts free access to aspects of their power while waiting for the perfect time to wrest control from their hosts and then seize control of the plane itself.

The way the possession works is that the receiver gains access to a fused eidolon that uses the host's class level to determine its strength. Eidolon's physical stats are used, mental stats are retained per usual. Ideas I'm playing with at the moment involve giving characters in the eidolon form access to a number of evocation spells as well as some bonuses from the existing demonic possession template depending on what demon the host is hosting. (Link (http://www.d20pfsrd.com/bestiary/monster-listings/templates/demon-possessed-creature-template)) When the demons start misbehaving (and perhaps when their powers are used to do good), hosts will need to make will saves to retain control of their bodies/use various abilities.

I suppose it may be simplest to just nix that last idea, but I'd really like to give the various possessions a sense of individuality. Regardless of whether I incorporate the demon possessed template, the question remains of how this affects CR/APL calculations. Any thoughts, suggestions, or answers?

[weckar]

Forcing mundane characters to become spellcasters is probably the worst thing you could do, imo. They chose to be mundane for a reason, after all.

The simplest way to handle this would be a generic progressive raising of ability scores (any two, depending on the specific demon), while lowering their will saves. The moment three will saves in a row are failed, the demon takes over.

[Malaqai]

This mechanic is being created mostly for members of the cult I mentioned in the OP, and I'd like to stick with the synthethist flavor for - well, flavor. My PCs come into this only because I've recently realized that three out of five characters are chaos muppets, and one of the remaining two is an amnesiac; they seem likely to side against the Paladins in my setting which means they'll have the option of receiving the cult's blessing.

Anyway, the big question here is how do I calculate CR/APL once these modifications have been applied?

[weckar]

Well, as you say, the Demons CAN take over when they want to. So, you may rule if your players are stupid enough to take part in this that they effectively become NPCs immediately, because it is impossible to tell when the demon is in charge. That would resolve that issue, at least.