Nagashizzar

By the shores of the Sour Sea, surrounded by the glittering desert of the Desolation of Nagash, is Nagashizzar, the Cursed Pit, mightiest fortress this world has ever seen. Built by the tireless labour of countless Undead things the castle rises nearly half a mile over the desert. It was excavated from the living rock of Cripple Peak and the mountain top is its highest spire. Hundreds of other great towers bristle from the mountainside. By night terrible green witchlights burn in their windows.

Nagashizzar is a fortified mountain pierced by countless leagues of corridor. Within thousands of chambers hundreds of Undead things wait ready to answer their lord's every command. Vast as this great army is, it is but a tiny fraction of the legions who once served the Great Necromancer. Four mighty gates guard the approaches to Nagashizzar, each watched over by mighty war machines of the most dangerous type - animated golems of bone, bolt throwers that fire the thigh bones of giants, catapults that throw screaming skulls and worse. The gates themselves are made from some nameless black metal which shines like burning obsidian and is ten times harder than steel. The pits beneath Nagashizzar extend down almost twice the height of the mountain, forming a huge honeycomb of galleries and mines where once Undead and Skaven toiled to find warpstone. These corridors are patrolled by untiring sentries from Nagash's Undead legions, who must be eternally vigilant in case the Skaven should ever return.

Within his audience chamber at the height of the peak the husk of Nagash sits on his throne of skulls. He has brooded here for over a millennium, waiting and planning, guiding his agents by the power of his thoughts. The only living things within this vast fortress are the disciples of Nagash. These madmen worship Nagash as a god and lead his cult until the day he re-emerges from his citadel to conquer the world. Now infused by the power of the Dark Magic that has surged through the world since the last Chaos Incursion, Nagash is almost ready to strike back at the world of the living.

Source

 * Warhammer Armies: Vampire Counts (8th Edition) -- pg.18