As with the other Mortal Realms, the realmstone of Shyish, the Realm of Death, tended to gather in greatest concentrations at the edge of the spherical realm. Over the centuries Nagash's forces laboured ceaselessly to carry the granular realmstone of Shyish to his lair at Nagashizzar.
By vitrifying this grave-sand under the intense heat of his dark magic, Nagash raised up glassy black monoliths and tall pyramids. Sigmar and his fellow gods of Order saw this as a labour of vanity, typical of one so monomaniacal as Nagash.
But these structures were designed for a darker purpose. Only when portents of doom and woe started to echo through reality did the God-King become aware of the magnitude of Nagash's plans. Though he reacted swiftly, it was too late to stop the dark ritual underway in Shyish.
By harnessing the Shyishan realmstone in such measure, Nagash succeeded in inverting the magical polarity of the realm. This grand act of cosmic engineering caused the greatest concentrations of Shyish magic to be drawn to the heart of the realm, rather than to its edge.
This slowly began to pull all the underworlds that comprised Shyish towards Nagash's capital, their ultimate fate to be ground down into the purest form of Shyishan or Death magic. Worse still, the explosive aftermath of that great collapse caused a wave of undeath to cascade across the aether into the other Mortal Realms.
The epoch-shattering explosion that blasted out from Shyish carried with it the spirits of the dead. Each soul was drawn along the impossibly thin silver cord that connected it to its mortal remains, emerging as a twisted Nighthaunt revenant from the grave of its former resting site wherever that might be in the other realms.
The necroquake had come, and with the unleashing of countless millions of ravening gheists, the path for Nagash's ultimate conquest of all Creation was laid wide open.
In one devastating act, Nagash became a force so powerful he had laid a claim to every one of the Mortal Realms, and the forces of Order fell before the hurricane of Undeath just as did the Chaos worshippers of the Dark Gods and the savage grenskinned tribes of Gorkamorka.