Katakros
"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.
"All proceeds as I have predicted."
- – Katakros, shortly after losing the majority of his army and having his body destroyed.
Full name Orpheon Katakros, the newest member of the Mortarchs, Katakros was revealed after a series of hints. The first was he had died and returned again and again, and had survived "The fall of the Storm God's hammer" meaning he had faced 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 mean only one thing: he was a new character nobody had ever heard about before and was being included in an "oh by the way" fashion. If he had a voice, it would be Jim Parsons The Big Bang Theory (or for a serious example, Stephen Lang).
History
Life
Hailing from the Realm of Beasts, in life he was an incredibly talented tactician and strategist, and thought it was his destiny to conquer and rule everywhere he went, Alexander the Great-style. However, he was bad at making friends due to being a bossy perfectionist and a callous workaholic. It also didn't help that in the army that he served, the vast majority of the upper command structure was made up of appointed aristocrats who did not take kindly to this bossy commoner upstart who had somehow risen all the way to become a general.
He died for the first time against Beastmen when he made a charge but no one was there to support his autistic ass. As a result he got plucked off his chariot and ripped to pieces by a Ghorgon, (just managing to deal a fatal blow to it as he died).
Death and Undeath
Katakros' manner of death gave him a disdain for mounts of all kinds, so in undeath he walks everywhere. His soul first went to the afterlife Ossia, a place where hard workers are rewarded by getting to do everything they enjoyed about their jobs with none of the downsides. Neighboring Ossia was another belonging to the same culture known as Necros, a place where the very same decadent aristocracy that had criticized and belittled Katakros' work lived out their afterlives being pampered and tended to as they had been in life. Seeing this disparity between Ossia and Necros, and since neither afterlife had a god they were on their own in disputes, Katakros began harboring an ever growing resentment for his spoiled neighbors, feeling that they were unworthy of such a reward in comparison to all that he had done in his life. Katakros' skill saw him rise to the position of general, but no one liked him there either because he was as callous and self-centered as ever - to the point that even Ossia's famously ruthless military officers were shocked - which reminded them of Nagash.
Speaking of the sociopathic super-skeleton, this was at the time Nagash had just started his omnoming of all the death gods and afterlives. By then Katakros led Ossia and wasn't keen to become Nagash's snack, so he annxed Necros under his rule (the blighters were so decadent they didn't even have a standing army). When Nagash's armies arrived, Katakros managed to defend his turf better than anyone else, since while none of the people liked Katakros, they could gauge his callousness and he got results (blah blah "better the devil you know"). It got to the point Nagash had to step in personally to curb stomp him. 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. Having been ordered to stop Sigmar by Nagash, Katakros took his forces to engage him despite thinking he could not actually win. After Sigmar smashed through his forces, Katakros, being "perhaps the greatest military strategist (living, or dead) the Mortal Realms have ever known" figured that he would be able to take Sigmar on in a one-on-one duel, guessing that that his glaive might be able to kill Sigmar because it was made using similar Warpstone methods to the weapon that slew Nagash in ages past. Naturally, Sigmar concaved the fucking idiot's skull with Ghal Maraz and won. Since this was before Sigmar thought about making the best warriors/leaders into Stormcast, (and apparently he didn't feel like turning "perhaps the greatest military strategist (living, or dead) the Mortal Realms have ever known" into a Sigmarine later), he threw the general into a Stormvault. Katakros spent his time mulling over tactics, stewing in his rage towards Sigmar and probably bouncing a ball against the opposite wall to pass the time. Unfortunately, rather than do something sensible like move it someplace Nagash couldn't easily access it, Sigmar trusted the enchantments to hide it from Nagash. Jump forward in time, Lady Olynder opened it up and here he is, ready to start the Tithe.
That being said, Katakros is the Mortarch of the Necropolis, and has his own sub-faction, the Ossiarch Bonereapers (also called the Ossiarch legions). He also commands his own personal legion of 10,000 Bonereapers within the Ossiarch legions called the Mortis Praetorians. 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 (but for some reason still less effective than grave guard, who cost only 10 points more than mortek guards). 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. 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 despite demonstrating a capacity for joy and self-deluding in the above quote.
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.
Wrath of the Everchosen
Currently Katakros is leading an invasion on the Allpoints, which means he was guaranteed to run into Archaon, the guy who beat both Sigmar and Nagash while Katakros beat neither. Joining forces with Olynder and some Nighthaunt, Katakros led his armies in an attack on the Allpoints while Archaon was away. After Olynder killed the Chaos Lord Archaon had left in charge, Katakros fortified the area around the Shyishian realmgate, making a fortress called the Arc Terminus, which looks like a massive skeletal hand reaching out from Shyish.
Then he led the Bonereapers against the Varanspire just as Archaon arrived back with his Varanguard. Unsurprisingly, all of his bodyguards fell before Archaon and Dorghar, so he took on the Everchosen himself and got a hefty does of the Slayer of Kings for his troubles. Convalescing back in the Arc Terminus, Katakros even tried to play it off as a just as planned moment by possessing another body identical to his old one and claiming he meant to lose, the plan being to learn more about Archaon each time they fight and eventually kill him. It's revealed that Katakros has as many duplicate bodies as the plot demands (which makes you wonder why Nagash didn't make all of the Bonereapers like that), so now he's heading right back to the fight so that Archaon can rip him apart a second time, except now he's without his glaive, the majority of his army is gone and the pain from Archaon's killing blow(s) won't fade even if he transfers bodies (for some reason he thinks this won't be an issue, despite that being Not As Planned).
It seems he has adopted Nagash's habit of declaring his own failures as mere setbacks, which debatably may hold water, given that both characters are able to continuously come back after getting pummeled or their plans getting disrupted. The main reason for this plan being to gather slowly gather intelligence on the forces of Archaon with each loss, despite the possibility that Archaon could turn it back on him and learn how Katakros' forces work. He already showed his best and brightest soldiers and generals in his initial put down of the Bonereapers, so now Katakros can build a strategy around that. Katakros isn’t trying to conquer the Eightpoints in a a swift decisive manner, rather he wants to stay dug in and play the long game, slowly building his repository of knowledge and army until he deems the time is appropriate. Why he thinks this is a valid strategy at the rate he's losing ground and troops is left a mystery, especially since Chaos can also resurrect their own champions and keep sending them back in if the Chaos Gods feel like it (and then there's the daemonic armies...), rendering the war of attrition he wants to get into impossible to win.
It is worth remembering that before Archaon returned, Katakros did secure a foothold for Death in the Eightpoints by capturing and fortifying the Realmgate to Shyish, as well as repel an army of Bloodbound sent to tear it down. Somehow he thinks he'll be able to keep control of it despite his plan to lose several times in a row.