Death Guard Occult Binder

Here is a special bit of heresy... A Death Guard Librarian. Now, how do I justify this special bit of crapping all over the accepted canon? Did you read the quick read "Daemonology"? Basically, Mortarion realizes he needs to learn more to stop daemons, and destroy psykers. A LOT more. What isn't said, but something I'm assuming, is that he is going to teach some of his "gifted" Legionaries