User blog comment:WhiteSeraph/Alternate Timeline Idea That I'm Really Psyched About/@comment-4051689-20110926161517/@comment-1932673-20110927153426

I agree, I think the Ultramarines have to some sort of presence, maybe they are stranded from the Imperium for decades instead, fighting these xenos, but a handfull of survivors (3000+ given the size of the UM legion) manage to escape and are possibly disgraced by their defeat, but are turned into an elite force, a little bit like a cross between the Deathwatch and the Grey Knights, dedicated to fighting these aliens.