A Primaris only Chapter. These guys willingly stayed in the Dark Imperium after the Indomitus Crusade. Otherwise no info beyond what YOU can make of them. The word "Umbra," is Latin for "Shadow," so their color scheme is probably black. This also makes them the "Shadow Knights." They are one of only three chapters of the Ultima Founding that have no other information. But, since the initial Primaris Marines were made from the nine loyalist legions and only three of those Legions haven't had a Primaris Chapter identified to them; they would probably belong to one of those three.