Rubric of Xanthism? (Xanthite Inquisitors use Daemons against Chaos) Supposing an Inquisitor figured out the Rubric of Ahriman. Perhaps he/she managed to reproduce it against a force of Grey Knights, but failed to keep them loyal to the Imperium. Could be a relatively good fluff explanation for the squad's psychic powers. The squad leader channels the powers.
Seem to recall that if a Vindicare assassin killed a GK squad leader, the squad can't cast magic anymore. Fits with Rubric Marine squad fluff, as they are led by a psyker, though the difference might be that Rubrick GK might be enchanted to not lose their leadership when the leader dies.
I know it's a bit of a stretch, but if you want to go with the resurrection/corruption theory, that's one way to do it.
Just my two yen
EDIT: GW has stated in the past, regarding fluff, that anything is possible on a small scale. A single force of something pure might be corrupted, and since the Inquisition lies about everything, you can be sure you'll never hear about it on official channels.
Last edited by Belthazor Aurellius; 07-07-10 at 10:32 AM.