(Reposted thanks to Reddit servers screwing up the original)
A lot of people have already mentioned that the zombies on Mars during the EE are visually one-to-one with the Crusader Ancients from the Crazy Place in Origins. What I haven't seen be noticed is that some of them are also donning the same logo that both the Ancients as well as the Keepers wear, the Black Sun.
Not to mention many of their robes are also a deep red similar (but not an exact match, the zombies' are less ornate) to the ones most of the Keepers wear in BO3.