so in the lore for the Hammers of Osterhagen, I've written down that they venerate "On the Great Skarosian Xenocide and the Emperor's Folly", written by "II" (obviously the Second Primarch), as some kind of holy text/operational manual, and that's why they do the whole "nomad-predation fleet" thing and steal everyone's taranium and generally just kind of aren't fun to be around, because that's what II wrote is what you have to do to fight the Daleks
but they're also not II Legion descendants, like that's something I'm being very clear on, they're probably actually Ultramarines, maybe with a bit of Dark Angels as their very Dreadwing-y heraldry suggests, and if they aren't, they're Death Guard, maybe World Eaters (the other Legions not the II involved in the aforementioned xenocide)