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 Katakros, 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.

His story was revealed in an episode of Stormcast. In life was an incredibly talented tactician and strategist, but not great at making friends. He died for the first time when he made a charge but no one was there to support his autistic ass. After being sent to one of the afterlife's of shyish he became the general there just in time to witness Nagash's birth into the mortal realms and his subsequent omnoming of all the death gods and afterlives. Katakros managed to defend his afterlife for longer than any other, to the point Nagash had to step in personally to curb stomp. Katakros, seeing the situation was hopeless, offered his services before he was stomped out and Nagash actually said yes, which should say a lot about this boy. He became the first Bonereaper, which was all fine and dandy until Sigmar invaded and concaved his skull with Ghal Maraz and locked him in a Stormvault, because this was before he thought about making the best warriors/leaders into Sigmarines and afterwards he apparently didn't feel like turning Katakros into one or moving his Stormvault someplace Nagash couldn't easily access it. Jump forward in time to the present and Lady Olynder opens it up and here he is, ready to start the Tithe.

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). He also commands his own personal legion of 10,000 Bonereapers within the Ossiarch legions, and apparently whenever one of his soldiers dies a new one is built back home and is then sent on a long journey to replace his destroyed soldier, making his army replenishment a logistical nightmare as it could take weeks to years to gain any reinforcements/replacements and a smart enemy could just attack them on the way, preventing them from ever reaching the main force. 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 Stormcasts 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 is at odds with his track record seeing as how he fought and lost three times in three major ways (admittedly twice it was against literal gods, and the first time he was betrayed and abandoned by his own men). 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 Stormcasts.