Nightmare of Drakenmoor

The so-called Nightmare of Drakenmoor was an infamous battle between the Chaos Dwarfs and the Skaven that took place in the year 2037 IC.

Such were the terrible arcane forces unleashed between the actinic flashes of the Skaven Lightning Cannon, the howling destruction of the Chaos Dwarfs' hell-furnaced colossus and the reckless devastating spell-craft unleashed by sorcerers and warlocks of either side, that a tear in the fabric of reality opened up over the battlefield and a howling storm of magic laid waste to both armies. Daemon-abominations were set loose and nightmare phenomena ravaged the lands a hundred leagues in all directions for almost a year before the arcane tempest at last died away. Both sides claimed victory.

Source

 * Tamurkhan: The Throne of Chaos.