The Apostles of Defilement are a ChaosSpace Marine warband which split off from the World EatersLegion following the Battle of Skalathrax, and as such are devout followers of Khorne. Consequently, the Apostles of Defilement are incredibly aggressive, prone to lashing out at their enemies even when not provoked. They have plagued the Imperial-controlled Cythraxan sector for numerous millennia, plundering and pillaging the numerous feudal and agri-worlds in the region. The sector would have quickly fallen at the hands of this warband were it not for the vigilance and valiant efforts of the Knights of House Aerynthus, whom have watched over the sector since the Dark Age of Technology. The Apostles of Defilement have thus been locked in an endless conflict of attrition with House Aerynthus for almost as long as the warband has threatened the sector, seeing the opposition of the Knight House as an affront to the glory of the Blood God and a challenge to their supposed right to claim dominion over weaker beings and their property.
Mysteriously, the Apostles of Defilement have remained a significant and ever-present threat to the whole of the Cythraxan Sector, in spite of suffering nearly constant casualties at the hands of House Aerynthus. Investigations conducted by the Ordo Hereticus indicate that the Apostles maintain their strength through a combination of the absorbtion of lesser warbands by way of force and by the creation of new Chaos Marines using stolen progenoids (where exactly they manage to obtain said progenoids is unknown; it is possible that they gain them through illicit backdoor dealings with opportunistic Adeptus Terra officials, but this is wholly speculative).
In 923.M41, the Apostles of Defilement made a daring strike directly at House Aerynthus' base of operations, the feudal world of Mythrim Primaris. House Aerynthus was caught unprepared, and suffered crippling losses, requiring first the aid of the Militarum Tempestus from the House's allies in the Schola Progenium, and finally a force of Space Marines from the Doom EaglesChapter to fully resolve the situation.