"Violence isn't always evil. What's evil is the infatuation with violence."
- – Jim Morrison
Krell was a Chaos Lord of a large tribe of Chaos Warriors dedicated to Khorne, living roughly 1500 years before the foundation of the Empire. He created a large kingdom somewhere in the future Empire and/or in the Chaos Wastes. After warring against human barbarians, he turned his attention to the realms of the Dwarfs, allying with Night Goblins. He attacked(and presumably conquered) the Dwarf fortresses of Karak Ungor and Karak Varn, but was defeated and slain while attempting to attack Karak Kadrin by an army led by Grimbul Ironhelm.
He was buried on the edge of the Chaos Wastes, and was re-discovered by Nagash while he was looking for his lost Crown of Sorcery. Knowing of his skills as a commander and warrior, he decided to resurrect Krell to serve him as one of the Nine Dark Lords of Nagash. He fought in the Battle of the River Reik, which he survived, escaping the battle and fleeing from Sigmar's victorious forces. With what remained of his soldiers, Krell laid waste to a swathe of towns and villages, till he was finally cornered and defeated at the Battle of Glacier Lake. His body was interred within a magically constructed tomb to prevent his return.
Even after Nagash's defeat, Krell was such an asskicker that even Sigmar couldn't do more than imprison him, whereupon he apparently "died" again. Centuries later, his prison-tomb was found by Heinrich Kemmler, the powerful and insane necromancer known as the Lichemaster, who broke Krell free and restored him to undeath. Originally, the Lichemaster believed that Krell was his servant, but with the coming of The End Times, Krell's true status - as Nagash's loyal Mortarch of Despair - has been revealed.
In the service of the Lichemaster
Heinrich Kemmler, the Lichemaster and a powerful necromancer, discovered Krell's tomb and managed to restore his powers and abilities by praying to the Chaos Gods. So it was that Krell rose from the dead once again, to fight alongside his new master. In the service of the Lichemaster Krell fought extensively against the forces of Bretonnia at la Maisontaal abbey, the Wood Elves during the 'Winter of Woes' and the Empire at Castle Reiksguard. It was at this last conflict that Krell dueled the infamous trollslayer Gotrek Gurnisson, actually managing to wound the dwarf, with the cursed power of his axe nearly gifting the slayer his long sought after doom.
During the early months of the End Times Kemmler and Krell joined forces with Arkhan the Black in his campaign against la Maisontaal abbey in Bretonnia, but the writers decided to shift around and make ambiguous (at first) who was the master or the servant for some reason. To this end Krell fought and slew the last Duke Tancred while Arkhan and the traitorous Kemmler battled to seize the staff of Nagash. The Lichemaster had chosen the uncertain future offered by the capricious and fickle Gods of Chaos rather than the sterile and static vision Nagash had for the world. After the so-called Lichemaster's destruction at the hands of the Liche King, Krell joined Arkhan. While Arkhan traveled back to Sylvania to resurrect Nagash, Krell distracted a following Beastman herd led by the Bray-Shaman Malagor that was bent on stopping him (even if they didn't exactly know why).
Mortarch of Despair
After Arkhan and his reluctant ally Mannfred von Carstein succeeded in bringing Nagash back to the world Krell, as one of the few surviving original Dark Lords, was made a Mortarch, one of Nagash's new commanders. Serving as one of Nagash's premier generals the new Mortarch of Despair was first sent to the Worlds Edge Mountains to report on the activities of another original Dark Lord, the vampire queen Neferata. Whether he was originally sent to recruit her or punish Neferata's past insolence is unclear, but it is known is that when he came across Neferata's legions battling a greenskin army in Skull Chasm he joined her in destroying the goblin forces. He then joined her in raiding a secret dwarf vault at Valaya's Gate. Here they worked together to destroy the forces of Karak Azul, with Krell himself slaying the legendary King Kazador in personal combat.
Following this Krell led the largest contingent of Nagash's three pronged attack into Nehekhara. Allied with fellow Mortarch Dieter Helsnicht, Krell faced the legions of King Phar of Numas at the Battle of Blight Water. He defeated but failed to obliterate the army facing him, so as he progressed first through the northern marshes and then the vast Salt Plains before finally the Great Desert, he was forced to fight a series of running battles with the great king as he marched towards Khemri over many months. Only the intervention of Helsnicht finally defeated the Numas army. Reaching Khemri, Krell led the assault against the vastly larger forces of Settra. Combining his own forces with those of Arkhan's elite guard from Nagashizaar, various traitor Tomb Kings and the mixed horde led by Mannfred von Carstein and Luthor Harkon, Krell started to attack the city gates. Despite putting up a fierce fight, Krell himself fell in battle, decapitated by a Tomb Scorpion. Despite his defeat, Nagash's forces were eventually victorious resulting in Settra being ripped to pieces, most of Nehekhara's armies bending the knee to Nagash and the destruction of its cities - with the exception of Nagash's Black Pyramid which mystically took flight and returned with its creator to Sylvania.
Krell was restored by the now fully powered Nagash. When a large Nurgle force invaded Sylvania, Krell marched to the province's defense at his master's order after the Mortarchs Mannfred von Carstein and Luthor Harkon failed to drive it back. Krell sent wave after wave of the more savage and bestial of Sylvania's undead to slow down the enemy horde, with no regard to the waste as this plan was co-developed with Arkhan to cull the less disciplined elements from their forces. Eventually however the daemon army still reached the Black Pyramid. Krell and his Doomed Legion anchored the center of the defending army and battled against the Nurgle daemons. Despite holding firm against the forces of the plaguefather Krell was bested in combat by the Great Unclean One Scrofulox, who proceeded to pick up the battered Wight King in one hand and eat him whole. The whole battle was revealed to be a distraction when a Skaven team led by Ikit Claw destroyed the Black Pryamid, dispersing Nagash's huge reservoir of death magic. In a rage Nagash destroyed the entire daemonic army, including Scorfulox and once again restored Krell to existence, even though he now considered him a failure.
Krell accompanied Nagash first to Athel Loren, but was ordered to stay outside Athel Loren with the rest of Nagash's army (only Arkhan and Vlad were allowed to remain). He was brought to Middenheim when the various armies of the incarnates were transported there by Teclis's desperate spell. At the battle of Middenheim Krell led the Doomed Legion into battle. However, his discipline was overwhelmed when he encountered Sigvald leading a combined force of his Slanneshi army and Throgg the Troll King's monstrous hordes. This triggered Krell's memories of life as a Khornate champion, specifically the anti-Slaanesh parts, so Krell charged Sigvald and engaged him in an epic duel. The fight went on for ages, the speed and murderous elegance of the Geld-Prince perfectly matched with the strength and relentless skill of the Wight King. Ignoring the battle around them, the two traded blows until eventually Krell managed a mighty blow that left a deep wound on Sigvald's face while also taking an eye and shattering Sigvald's sword. Vain beyond mortal understanding, the deforming injury drove Sigvald into an insane fury and he finally overcame Krell with a mindless series of frantic, savage blows with shield, fists and his broken sword, senseless to any further damage to himself in the process. Krell was defeated once more, but this time Nagash had neither the energy or inclination to restore him before the world ended.
It's worth noting that despite being a loyal Chaos Lord of Khorne in life, after death Krell seems to have soured on his old master and completely embraced Nagash in unlife. He is literally the only Mortarch who never second guesses, defies, disagrees, criticizes or has any doubts of Nagash or his leadership. Even Arkhan at times has some criticisms of Nagash but Krell? Never even thinks of being the slightest bit disloyal to the skelepope even as part of the faction with one of the largest counts of turncoats to Chaos of any of the players involved during the end times.
Age of Sigmar
So far it doesn't look like Nagash saw fit to restore him in Age of Sigmar. HOWEVER, there is now a unit called "Wight King with Black Axe" that uses Krell's model and functions just as he did (minus the Mortarch of Despair rules), so maybe Nagash brought Krell back in pieces and each one is a fragment of his asskicking ability. A glimmer of hope remains as there is a character named 'The Dark Lord of Despair' in the Grand Alliance: Death book. His armies are rumored to be relics from the age of myth, but they say the lord himself is FAR older than that...
Another snippet is shown in the first of the Eight Lamentations book trilogy, Spear of Shadows. In it Chaos champion Ahazian Kel finds a shard of Gung in a crypt in the Realm of Shyish. Inside is a hulking skeletal corpse that is described as having faded crimson armour covered in bat winged skulls, with a heavy, horned helm, topped by a frayed crest and with a great, black bladed axe. So yeah, that's literally Krell.
During the Stormvault campaign it's revealed that an ancient ally of Nagash was locked away in one of the Stormvaults, but was freed by Lady Olynder. This ally is said to be one Sigmar had a beef with in the past. While there are several it could be, and Krell fit the bill opening the possibility that it could've been him. Later it was revealed to be a new character called Katakros, a cruel tactical genius leading an army of skeletons and bone golems in Nagash's name. Despite this, some Krell fans still hold a torch.
On the Tabletop
Krell is a melee monster, as you'd expect of an Undead Champion of Khorne. Traditionally, he sports the Armor of the Barrows (magic heavy armor), and the Black Axe of Krell (magic greataxe that causes continuing damage). His latest version of stats as the Mortarch of Despair gives him some new tricks, like gaining Heroic Killing Blow if he's in the same combat as Nagash (hilariously, due to RAW you can use that to kill Nagash), and regaining wounds when enemies fail Fear, Terror and Break tests within 12".
In Total War Warhammer
With the Old Friend free-DLC Krell is back in digital form! Heinrich can summon him on the battlefield for a limited amount of time (Krell can last longer by fighting or by unlocking skills which lengthen his stay in the battlefield or make him permanent). One can wonder how TW3 would affect both Kimmler and Krell!