Katakros

Dude gave himself a bulge bigger than most heads.

"I take no joy in this. Nor do I despair. It is merely something that must be done. All we ask is payment of the Tithe. These people had their chance! They chose their fate! Perhaps I am wrong though... perhaps I deceive myself. For in truth, there is... some joy in this."

– Katakros on his 9 to 5.

Full name Orpheon Katatros, the newest member of the Mortarchs, Katakros had died and returned again and again, and had survived "The fall of the Storm God's hammer" meaning he had either faced the Celestant-Prime or Sigmar Himself and was defeated. The other hints were that he died a general defending his birth city, and came back in death as an Emperor, and his promotional material was visually similar to Drachenfels, while some hopefuls thought that there was Tomb Kings influence in the army he led. This also came after the plot focused on Lady Olynder working to free an ancient evil that Sigmar had beef with in the past, with many people thinking that was either Drachenfels or Krell.

All of this could only mean one thing: he was a new character nobody had ever heard about before and is being included in an "oh by the way" fashion.

Katakros back in the Age of Myth.

That being said, Katakros is the Mortarch of the Necropolis, and has his own legion, the Ossiarch Bonereapers (also called the Ossiarch legions). Unlike most undead characters inhabiting thin bone-bodies that are inevitably broken, Katakros is a soul housed within a fucking massive bone-construct (one that has a ridiculous bulge that would make David Bowie from Labyrinth blush) and also leads a legion of bone constructs, each housing several souls to make them more effective then brittle skeletons or slow-ass zombies. Apparently Nagash saw Sigmar's Sigmarines and wanted to steal the gimmick for his own in revenge for all those tasty souls he will never own.

Katakros is also described as being "perhaps the greatest military strategist (living, or dead) the Mortal Realms have ever known", which means he probably isn't, given that we've never heard of him before this point. He is also seen as the paragon of Nagash’s dreams for the Mortal Realms; a consummate professional focused solely of the efficient completion of tasks, unfettered by the weight of emotion or free will.

Also, he looks like a Primarch cosplaying as an Alien: Covenant Space Engineer. Because apparently THAT is a successful enough idea to rip off, even if it does match the alien molded bone look of his skeleton Ground Marines.