Battle of Kornovin
The Battle of Kornovin was a clash in 901.M41 between the Death Guard Chaos Space Marines, led by the Daemon Primarch Mortarion himself, and the Grey Knights led by Supreme Grand Master Geronitan. Mortarion had previously ravaged a large number of Sectors, but the entirety of the Grey Knights only chose to oppose him on Kornovin when Geronitan felt the time was right.
During the struggle, Geronitan was slain by Mortarion and the Grey Knights were forced to withdraw to their final stronghold on the planet as the Daemon horde advanced. During a psychic conclave between all the Brotherhood's Grand Masters, Kaldor Draigo was approved as the new Supreme Grand Master of the Grey Knights. Draigo was then psychically transferred with the knowledge of Mortarion's True Name, the name the Emperor himself had originally intended for him. With this weapon in hand Draigo met Mortarion on the field and after battling through his Deathshroud bodyguards faced the Primarch himself. Draigo proved no match for the Daemon Prince, and on the verge of defeat put his last once of strength into a final psychic bolt. The blast made Mortarion drop his mental guard for a moment, giving Draigo the window he needed to speak the Daemon Prince's True Name. Mortarion's body was shattered in that moment. Draigo ripped Mortarion's heart from his body and carved Geronitans name into it. His material body destroyed, Mortarion's spirit was sent to the Immaterium. The Primarch has never forgotten the insult Draigo handed him that day.