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.

The newest member of the Mortarchs, the trailers stated Katakros had died and returned again and again, and had survived "before the Storm God's hammer" (which means either he survived getting hit by it, or was alive before Sigmar recovered Ghal Maraz, or both). 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.

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.

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 thing bone-bodies that are inevitably broken, Katakros is a soul housed within a fucking massive bone-construct (one that has a ridiculous bulge he presumably can't even use) 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.

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.