Black Tower of Arkhan

The Black Tower of Arkhan was built by Nagash's trusted vizier at the height of the Reign of Terror. Isolated deep in the Crack Land, the Black Tower offered Arkhan a refuge where he could develop his mastery of Dark Magic. Constructed in union with the accursed Black Pyramid, Arkhan's infernal spire and the lands that surround it are saturated with magic, and many vile things dwell in its shadow. Following Nagash's defeat by the Army of the Seven Kings and Arkhan's mortal-death, the Black Tower was abandoned, and for centuries it remained a haunted place, shunned by all. Only evil spirits stirred inside its catacombs. Upon Arkhan's return to Nehekhara, he raised an army of Skeletons to guard against his Tomb King rivals and carved out a realm of his own. Arkhan rules from the Black Tower, maintaining an uneasy truce with monarchs of Nehekhara through the power of dark sorcery. Whatever his true motives, Arkhan the Black is a powerful ally for those who can afford his price...

Source
pg. 19
 * Warhammer Armies: Tomb Kings (8th Edition)