The Skull Lands War was a conflict fought in the Realm of Chaos in the Immaterium as part of the Great Game between the major Chaos Gods. At the zenith of one of the Blood God's reigns of dominance in the Great Game, Tzeentch encouraged Nurgle to invade the realm of his brother Khorne, assuring him that both his own daemonic legions and those of Slaanesh would aid him.
Convinced, the Great Corrupter sent his most faithful servants to the Skull Lands, instructing them to take the bounty of his garden with them. Sure enough, powerful Tzeentchian illusions drew many Blood Legions away to chase phantom armies, rendering Khorne's armies vulnerable to the combined forces of his rivals. So great were the Blood God's losses in the ensuing conflict that the minions of Khorne were pushed back to the very walls of his keep, the Brass Citadel.
All around that indomitable fortress, vast swathes of the Garden of Nurgle had sprouted, the land ploughed and seeded by Horticulous Slimux and watered by Rotigus the Rainfather. With victory seeming certain, a supposedly stray spark of Warpflame from a Herald of Tzeentch ignited Nurgle's flora, dried out as it was by the desert heat of Khorne's underground forges.
The resultant inferno created a wall of flame around the citadel that utterly engulfed Nurgle and Slaanesh's daemonic legions, and began to spread uncontrollably back to the centre of the Plague God's domain. Only by Rotigus' quick thinking was the blaze prevented from reaching all the way back to his master's manse, as he called forth a bigger variant of Nurgle's Deluge than he had ever previously manifested. Seeing the towering flames around the citadel, Khorne's Blood Legions realised they had been tricked, and returned to repel the last of the invaders.
- Codex: Daemons (8th Edition), pg. 56