Ah, didn't play the rest of them then, where it turns out that the chapter master of the Blood Ravens has fallen to chaos and half the chapter went with him.
Also they've got some skeletons in the closet wrt who their "lost" primarch is* and how much heresy they've been up to. And how all the other chapters' relics mysteriously turn up in their armoury.
* Given that they're notable for how many psykers they have the speculation is that it's Magnus the Red.
I also think the series will probably be about the Inquisition, because that gives the most license to fiddle with pretty much whatever aspect of the universe it wants.