The Mythos Angelica Mortis is a tome of ancient knowledge listing the fraternity of twenty Space Marine Chapters, collectively called the Astartes Praeses, which had been created for the sole purpose of guarding against incursions from within the Eye of Terror following the Great Scouring. In the current 41st Millennium, seventeen remain dedicated to this duty. The Archenemy had annihilated one Chapter and, more shocking still, two others had been branded Excommunicate Traitoris.