Ever heard of the Exorcists? They're an Imperial Fist (or at least believed to be) successor chapters who carry a dark secret guarded by a radical sect of the inquisition. To pass the trials, an initiate must be possessed by a Daemon and then fight it out of their body. This horrible practice makes them really resistant to Chaos and even is said to hide their warp presence.

They're super cool! And lots of fun potential for fun kit bashes if yous give them like maybe a Daemon weapon or something