Page 15 Gk 5th edition codex In early 901.M41, during the Battle of Kornovin, Mortarion slew Geronitan, the Supreme Grand Master of the Grey Knights Chapter. In his place, the Grey Knights elevated the hero Kaldor Draigo amidst the din of the battlefield and Draigo immediately vowed vengeance against the Daemon Primarch. Alone and unaided, Draigo smashed his way through Mortarion's bodyguard of Plague Marines, struck the ancient Primarch to the ground and carved Geronitan's name on the Daemon Prince's diseased heart. Though Mortarion ultimately escaped, it would be many long years before he could enter the mortal realm once more. Draigo's insult would not soon be forgotten, as the Daemon Primarch vowed his vengeance against the upstart mortal.
See and despair you mighty who fucking said it said nothing about this. Bah. There it is in all it's vileness.
Proves only one thing to me, the Matt Ward-apologists are willing to lie just to make his fluff a little less vile. Or at least overlook.