Changes

[[Horus]] found Mortarion more difficult to bring to his cause than either [[Angron]] or [[Fulgrim]], and for a time it seemed the Warmaster may have failed to convince the Lord of the Death Guard. However, Horus at last found a chink in Mortarion's armour; he was beginning to see the Emperor as having been corrupted with power and now was just another tyrant drunk with power. Indeed, Mortarion had become disgusted with what exactly the Emperor was, considering him a [[Warp]]-tainted "aberration" like his tyrant adopted father. Horus also used Mortarion's distrust of the Warp to his advantage, arguing that the Emperor had used the [[Warp]] in the creation of the Primarchs. Horus eventually used these doubts to bring Mortarion to his cause.{{Fn|6a}} Mortarion led his Legion in their betrayal of the Imperium at the [[Battle of Isstvan III]] and [[Drop Site Massacre]]. He later came to blows with [[Jaghatai Khan]] on [[Prospero]] after failing to convince him to join with them in rebellion. During the fight the two Primarchs were able to challenge the other, with the Great Khan proving faster and the Death Lord proving more durable.{{Fn|9}} Following the battle, Mortarion abandoned his pursuit of the White Scars and instead began a spiteful purge of the systems surrounding [[Prospero]]. During the purge, Mortarion encountered a [[Daemon]] possessing the body of a woman and was forced to kill it using his innate [[Psyker|psychic]] abilities despite it being against Imperial (and by this point, his very own) dogma. Realizing that the Emperor had lied to him about the [[Warp|Empyrean]], Mortarion vowed to master it.{{Fn|10}}

By the time of the [[Battle of the Kalium Gate]] late in the Heresy, Mortarion's forbidden knowledge had grown considerably. He experimented with both [[Xenos]] and [[Chaos]] artifacts, vowing to remain pure through understanding. At this same time he was tasked with [[Horus]] to find and destroy the [[White Scars]], who had been harrying the traitor lines since the events on [[Prospero]]. Mortarion was initially reluctant, but was moved by Horus' declaration that he was the only Primarch the [[Warmaster]] still could rely on and was eager to settle the score with the Khan anyway. In the subsequent [[Battle of Catallus]], the joint Death Guard-[[Emperor's Children]] fleet led by Mortarion cornered the White Scars fleet at the [[Dark Glass]] artifact. But as Mortarion boarded Jaghatai's flagship, the ''[[Swordstorm]]'', he discovered the ship abandoned save for a contingent of suicidal [[Sagyar Mazan]] warriors. Worse still for the Primarch, the reactors of the ''Swordstorm'' had been set to overload and the Khan had since evacuated to the Battleship ''[[Lance of Heaven]]'', which was leading the escape of the loyalist fleet into the [[Webway]]. Unable to escape thanks to the Sagyar Mazan and the vessels reactivated [[Void Shield]]s, Mortarion led the massacre of the Scars suicidal rear guard while directing his fleet to combine its fire and reduce the ''Swordstorms'' shields. When the shields were depleted shortly after he personally killed [[Torghun Khan]], Mortarion barely [[teleportation|teleported]] off the ''Swordstorm'' before it exploded. Following the battle, Mortarion realized that his divided fleet had hampered the war effort and ordered [[Eidolon]] to find [[Calas Typhon]] and his splinter fleet.{{Fn|13}}