Actually there is this SOB gal that managed to gain so much power she has the inquisition and the forces of chaos after her . If I am correct there is a novel and/or comic covering her story. She is quite OP by killing several grey knights (she had no option) and even killing a blank imperial assassin (yup the ones wearing big skull masks) . Still in my opinion she is no way as OP as this Abomination. You are right this should have clearly remained in the assassin domain and not pulling Astartes into this whole mess. In fact there was quite some potential to see other "houses" introduced. I bet she was pissed off about not able to consume the marines due to the power armor.



If I was Lysander I would have told the Assassins to scram and leave me be...I have planets to liberate .