The Knights Of Magnus are a warband that split off from the Thousand Sons, and unusually, this was done with Magnus the Red's blessing. Lead by the Chaos Lord Evalax (Who styles himself Lord of the Dead), who commands his warband onto greater heresies from the Space Hulk Apostle of Ruin. While they do have Magnus' blessing, they are viewed warily by the other Thousand Sons, for Evalax's goals and the fact he has an impressive amount of knowledge regarding Rubric magic. But to gain a place among the Chaos Gods pantheon, and his formidable psychic powers lend this worry some credence, as he is capable of resurrecting as long as there is a Scion Of Evalax.
They are mistrusted by the Night Lords, as it is believed (And true) that the Knights of Magnus have kidnapped some of their brothers, these kidnapped brothers are turned into Warpyres to better serve the Knights of Magnus. While the Imperium can only speculate at Evalax's warbands goals, they can mean only trouble and damnation for the Imperium, for he has been responsible for the fall of Inquisitor Valdria and her entire network of agents.