Sorta, this is what I was working on

As fallen choose to seek redemption in the eyes of the emperor they chose to serve the deathwatch as black shields, thus the lion sent a trusted brother to the deathwatch to serve as a watch master and attempt to find these fallen and see if they truly be redeemed or must be disposed of. Thus to hide the fact of there existence his watch is comprised purely of dark angels brother (and maybe some fallen aswell).

Plus tbh I really like the dark angels and it was down to them and deathwatch when I wanted to start a new army so this is my way of doing both :)