>>96224387
tl;dr Malcador creates the Grey Knights for the Emperor after attempting to "save" a Thousand Sons loyalist marine (secretly using the "pure shard" of Magnus to create Janus who, in my opinion, was going to be an alternative to Magnus to occupy the Golden Throne. )

Janus is found wanting, to occupy the Golden Throne and as the """Primarch"""" of the Grey Knights, but the flesh change scoured gene seed he possesed is reserved as the foundational stock for the Blood Ravens. The Blood Ravens are established as a gift from the Emperor to Malcador in recognition of the establishment of the Grey Knights, with the Blood Ravens serving not only as the eternal extension of the Sigilite tradition that Malcador carried but forming the de facto chamber militant of the Ordo Hereticus during the time of the scouring acting as the Inquiitorial/mind wiped Astartes chapter designated as such by Malcador "the bureaucrat" to carry out Edicts of Obliteration and further prosecute the Sigilite's "Secret War" against Rogal Dorn. Eventually the Sisters of Battle are established as the Ordo Hereticus chamber militant the same years as when the Blood Ravens are released into the Imperium at large.