Possessed foes, My group keep out! You know who you are... Options

[FuelDrop]

Ok, here we go again!
It looks like I've finally got the chance to do some GMing (you know all those other threads that I've started about this? Every time something has come up and I've ended up a player. I'm kind of like an inverted CanRay!). The run is investigation (the group is keeping a low profile and trying for some semi-legal work after some flashy recent antics), tracking down a stolen holy book kept hidden by a local church (They've recently had a security shake-up, and their boss isn't sure who to trust any more). This book was taken by a cult who believe themselves to be the knights templar reborn, and who intend to seal the book away from the world. shortly after the PCs track them down another cult will attack and steal the book, intending to read it and summon some horrors for a lovecraftian apocalypse (AKA summoning Cthulhu.)

Now by the time the PCs track down this final cult they have already cracked open the book and, as a result, everything in the immediate area had been possessed by low-powered (force 2-3) spirits of assorted types. Tentacle mutations abound, though mostly hidden beneath lined cloaks... though seeing possessed dogs with many-eyed tentacles where their heads should be is going to call for composure checks.

Now as far as I know there are no official stats for the horrors, so I was going to use toxic spirits EG Abomination spirits, with most of the cult rocking a force 3 possession while the cult leader (The guy who opened the book) rocks out with a force 7. I was going to give the cult's guard dogs force 1-2 possessions as well, then use them as cannon fodder to slow the PCs down.

Thoughts or suggestions?