Nagash
Nagash is the First Necromancer and arguably the most evil character to ever curse the Warhammer Fantasy world. Ever. He is also fucking badass and will go in hand-to-hand combat with the likes of Sigmar. His badassery sometimes interlope with how much of a fucker he is, in other words, he would have had no problems emigrating to Warhammer 40000 setting.
Background
Nagash started as a hierophant consecrated to the gods of Nehekhara while his brother was the priest-king of Khemri. This was because, in Nehekhara, the firstborn sons of the nobles were given to the temples as priests and the second sons became the kings. Pissed off about this, he decided he wouldn't accept his fate and started investigations on how to increase his magical power. Unfortunately for the khemrians, he got his hands on some Dark Elf sorcerers who'd been taken prisoner. From them he learned about the winds of magic and also figured out how to increase his own life expectancy. He then proceeded to brutally kill them and consume their souls when they'd outlived their usefulness. When someone can out-evil and out-betray Dark Elves, you know they're fucked up.
After awhile and many experiments which demanded human sacrifices, and sick stuff Nagash created an elixir which allowed him to stay alive, this elixir would be the basis for the creation of the Vampires. Nagash killed his brother, and sorta mummified alive his wife and then started a war on Nehekhara which ended in the complete annihilation of all life there. Unfortunately for him the Skaven sabotaged his plans of world domination and he got chopped to pieces by the last king of his former nation. When Alcadizzar killed Nagash, it wasn't through any fail on Nagash's part. It was basically the ultimate sacrifice - killing Nagash cost Alcadizzar literally everything. His kingdom - the largest empire in the world - was killed to a man, his family died, his sanity was shattered and the weapon he needed to use to kill Nagash was so potent it killed the wielder as well. That weapon was created by the Skaven who had realised Nagash needed to be stopped so badly that the Council of Thirteen actually stopped their infighting and co-operated to create it
His story, however, doesn't end here; he went back after a couple of centuries (the Skaven had vaporized his remains so it took a lot of time for him to remake himself) and attacked the newborn Empire, during the final battle he fought in a duel with Sigmar, and nearly defeated him. Sigmar, realizing what was at stake went on a Humanity Fuck Yeah! rampage and finally crushed Nagash's skull with his hammer, the spirit of Nagash fled the battlefield and went back to his fortress were he recovered, having learned that the world now has powers capable to match him. Even Sigmar at the height of his power only just managed to defeat Nagash, and even then only by wearing Nagash's own crown to protect him from Nagash's magic. A crown that had pretty much sent Sigmar insane the last time he wore it. No one knows for sure what's he gonna do next but considering his profile it won't be anything pretty.
Why Nagash is so evil
While most evil characters on the game have done their share of bad deeds Nagash has an special place amongst them, the following lists illustrates how sick this fuck is:
- Entombed his own brother alive and stole his wife.
- Usurped Nehekhara's throne then starved and exploited his own subjects to death, along with nearly bankrupting the kingdom, to get his necromantic works done.
- Turned said wife into an agony-ridden walking corpse and kept her that way for centuries. The process he used to accomplish this involved tricking her into drinking the blood of her murdered son. The son who was also Nagash's own nephew.
- Captured the spirits of his enemies keeping them in eternal torment.
- Started a war which destroyed many of the Nehekharan cities and killed a great deal of the population.
- Broke the covenant between the Nehekharan gods and their people, not only removing the divine powers of the Nehekharans but ensuring that after death they wouldn't be able to go to their gods and would have to stay in a nether dimension forever. Especially jarring if you remember that he used to be the High Priest of their religion.
- Indirectly corrupted some of the nobility of Nehekhara, who became the first vampires.
- Turned a whole tribe of his followers into ghouls.
- Used his loyal vassals as tools in a terrible incantation to make himself a magic armour and then devoured their souls.
- Started a new war against Nehekhara which cost more thousands of lives.
- Used a horrible plague spell to annihilate the entire Nehekharan civilization after losing the war against them.
- Raised the Nehekharan dead turning them into millions of undead minions with the idea of annihilating all life in the world.
- Almost destroyed the Empire and nearly crippled Sigmar in a duel by using a poisoned blade.
- Cursed the Vampires with a vulnerability to Sigmar's power after the assholes were too self-absorbed during the war with the Empire to help Nagash out.
- Corrupted the dragon's graveyard and created the zombie dragons
While some of the deeds on this list may have been done by your Skaven or Chaos Lord there is a basic difference between them and Nagash, the former usually do this either to advance their entire race, clan, tribe or to appease and/or promote their gods. Nagash on the other hand didn't care about anyone, despised the gods and had no empathy for his people. He fucked up his own nation and a large section of the world for his own personal gain and, so far as the fluff goes, he has never cared or done anything for anyone other than himself, with his ultimate plan being quite literally to turn everything into undead with no will under his command. To be fair, he honestly believes this to be the best thing for the warhammer world. And he might be right. Chaos has a hard time corrupting the undead, and Nagash has already managed to steal one of Khorne's favoured champions (Krell).
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All that said, Nagash is a knock off attempting to one up the original; Drachenfels. He's not really evil, he's just trying too hard. The idea of Nagash copying Drachenfels is doubtful at best. absolutely correct. It is true in the setting that Drachenfels is much older than Nagash, the character of Nagash was put into the setting of the Warhammer world before Drachenfels was. Drachenfels dates back to 1989, Nagash 1994. In addition, Nagash doesn't use armies of daemons, has no respect for the Chaos Gods and there is no evidence in canon that the two of them have met or even had contact with each other. And the editor completely missed the point, Nagash was introduced to supplant Drachenfels as the setting's 'dark lord' hence the trying to hard with his over the top evil.
Nagash was actually a special character back when it was just "Warhammer Armies: Undead" and all the dead boys were united in one armybook. Despite being described as "a pale shadow of his former self" he was an unholy rapetrain - a statline with the lowest stats being 6's (init and attacks) and everything else being a 7. Add in a completely unmodifiable 4+ save against everything (including any and all spell effects), a sword that gives him +1 str and lets him use any wounds he causes to heal himself and being one of the most powerful mages in the game making him pretty much unstoppable. (Unless you threw a High Mage at him with Drain Magic and Banishment which resulted in epic lulz.) Even if Games Workshop was willing to advance their storyline, Nagash getting off his bony arse and doing shit would be a game ender. Nothing short of the direct intervention of gods would stop him and even then, the world would be a graveyard.
Functionally, Nagash is Warhammer's big bad to 40k's Abaddon. Except Nagash is notably more competent. When Abaddon tried to conquer the galaxy, he tried 13 times and basically got nowhere. When Nagash decided to conquer Nehekara, only one man survived. When Nagash decided to raise up every person who had died in Nehekara over the past 1500 years, he succeeded.