...huh. Well I have no idea what this is about, beyond what somebody on Facebook figured out from the cover, it's seemingly about this guy;
Covenant - Lexicanum
Which would make sense since Horusians are a faction who believe that the Emperor can be resurrected via a Divine Avatar, like the Thorian belief, but where they become Radicals is that they think Chaos can be used to achieve this, and that Horus in his final days was an example of such an Avatar. Covenant is an Inquisitor devoted to rooting out beliefs like these and putting their practitioners to the torch.
So the question is, is this a new series?? Because if so I would heartily approve of that, we need more Inquisition stuff and we've had two Inquisition series so far about Inquisitors dealing with the enemy without, it would be very interesting to read about an Inquisitor whose job is to investigate his own kind.