House Aerynthus was an Imperial Knight House based on the agri-world of Mythrim Primaris. Since the Dark Age of Technology, the Knights of House Aerynthus watched over the predominantly rural worlds of the Cythraxan Sector, being accepted into the folds of the Imperium once during the Great Crusade and again in the 33rd Millennium after having fallen out of contact with the Imperium in the early 32nd.
Since the 36th Millennium, House Aerynthus clashed frequently with the Apostles of Defilement, a large splinter warband of the fractured World Eaters traitor legion. This rivalry came to a head in 923.M41, when the Apostles made a daring full-on assault on the House's homeworld, requiring the aid of both the Militarum Tempestus and the Doom Eagles Space Marine Chapter to fend off. Since the attack, the Apostles of Defilement have not been sighted again in the Cythraxan Sector, though it remains to be seen whether the warband was truly destroyed or if it is simply licking its wounds.