Undead (Warhammer)
The Undead of the Warhammer Fantasy Tabletop Wargame (Games Workshop ltd.), were introduced to the game in its very earliest editions. The term itself can refer either to the undivided and all-inclusive army—ranging from ghosts and vampires to skeletons and mummies—or to the separate components which make up the two: The Tomb Kings of Khemri, and the Vampire Counts. Up to and including the fifth edition of Warhammer Fantasy Battle, the Undead represented the combined forces. For the sixth edition, Games Workshop divided the Undead into two separate armies to represent the chaos which Nagash created when he cast his Spell of Awakening to begin his assault on the lands of the living.
As in-game legend goes, Nagash was a priest in the courts of the land of Nehekara, and after learning Dark Magic from prisoner Dark Elves, developed his own twisted form of Magic which he termed Necromancy. This magic gave him the power to command the dead, and ultimately he cast a powerful spell from his tower, with the intent of raising all the dead of Nehekhara, thus creating a titanic army of mindless Undead. The recent dead joined with the enslaved Tomb Kings of ancient Khemri to begin their march to Nagash's tower. They would not complete their journey, for Nagash was slain through the machinations of the Skaven and their pawn, the escaped captive King Alcadizaar. The ratmen, who had grown jealous of Nagash's hoard of Warpstone, gave to Alcadizaar a sword which could cut down the mighty sorcerer: The Fellblade. While the act did not destroy Nagash, it banished him from the material world for more than 10 centuries, collapsing most of Undead horde that his sorcery had raised, and leaving the Vampires to pursue their own ends as his unwitting progeny. With Nagash's death, the Tomb Kings retained their unlife, but had regained their individual will.
Upon Nagash's first return from death, the Tomb Kings revolted against him, and swore vengeance upon his head and upon any of his followers. This is the reason that the two types of Undead exist—because of their loathing of Nagash and his evil magic the Tomb Kings have separated themselves from such atrocity. No necromancer is safe in the deserts of Araby or Nehekara, for the kings of old lie in wait to destroy all of Nagash's minions.
Out of the two offshoot armies, the Vampire Counts have access to strong heroes and much of the troops from the old Undead, making it more of a successor to the formerly unified army.
Read more about Undead (Warhammer): Distribution, Development of The Undead in The Warhammer Setting, The Legions of Undead, Undead Characters, Liber Necris