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 how to shut down daemons and psykers themselves. Mortarion can't be everywhere at once! So, I created the Occult Binder. Not there to use psychic powers themselves, but to shut down those who do!
The model itself is a stock MkIII resin, with a Grey Knight force staff. I just cut the skull out of the center.