How about this: Nagash musters the greatest armies of undead ever seen. He gathers his closest and most powerful sorcerors imbueing them with immense power and divides his super-undead warrirors into twenty legions. He creates or promotes 20 vampires/ ghouls/ tomb kings and grants them riotous power to be his generals and sends them forth to conquer the world.
Except, mishap strikes: Upon hearing of his plan, the powers of chaos, desperate to stop their arch nemesis and the only one with the power to truly end them, manage to create some kind of dimensional storm or warp anomaly, scattering his generals while they are still being perfected.
Nagash sets out on a crusade to find his scattered 'children', becoming reunited with them one by one and finding each changed by the circumstances they found themselves in.
After a while, he hands control of his armies to Warmaster Arkhan, and retires to Nagashizzar to work on some unknown creation of immense and arcane power.
On the brink of victory, the other forces of the old world all but broken before the undead, Arkhan betrays Nagash, claiming dominion of the undead for himself and convincing fully half of the super-undead legions to rebel with him. A bitter civil war of the undead follows.
Nagash remains under Nagashizzar, working on his creation.
In the final showdown, the forces of Arkhan reach Nagashizzar and Nagash Faces Arkhan, now a twisted parody of his old self, but is unable to kill his former friend and ally, and perhaps the only one in the entire old world Nagash might come close to calling an equal.
The two fight and although Nagash eventually destroys Arkhan, in his final moments Arkhan curses Nagash, forcing him to spend the rest of eternity bound to the device he was working on beneath Nagashizzar, the Throne of Bone, which was the key to finally sealing the forces of Chaos away forever.
Weakened to the point of un-undeath, kept undead only by the arcane energies of the throne, Nagash becomes ruler of a crumbling undead empire, beset upon on all sides by the living and the damned, twisted races long thought crushed coming to the fore to feed upon the dying empire of the undead.
And so it is for ten thousand years, the corpse emperor silent upon his throne, the great empire of the undead desperately clinging to it's former glory but losing ground day by day.
In the grim far future of the old world, there is only war.