Aval.
It was a Knight World of the Imperium, stationed at Three Gates Star.
Three Gates Star was an exceptionally important planet in the Ultima Segmentum.
The region was an arsenal formed by linking and assembling three star systems together, with each system containing three Forge Worlds.
There were nine forge planets in total, producing parts for the Imperium's main battle tanks. However, with the emergence of the Great Rift, this world was completely cut off from the human Imperium, and these nine planets became a prize Chaos most desired to plunder.
In the early days of the Great Rift, certain traitors had already discovered that abnormal psychic awakenings were running rampant throughout the Threefold Forge Cluster. Thus, the Thousand Sons Legion began supporting Chaos cults there on a massive scale, using these corrupted cults to further corrupt the lofty administrators and even the planetary governors, ultimately seizing control of the entire cluster for themselves.
After all, this planet had nine forge worlds, and nine was the sacred number of Tzeentch's cults. Clearly, these planets embodied some cosmic law and belonged to Tzeentch.
However, just as the Thousand Sons' sorcerous warbands of Tzeentch set their sights on this planet—
Abaddon's Black Crusade had suffered a setback, and Nurgle's fury over the Angel's soul being torn apart and forcibly stolen from his grasp shook the entire Garden. When news of Grandfather's wrath spread from Nurgle's Garden to the Death World where Mortarion resided—
Mortarion gave the order for his Death Guard to help Nurgle reclaim the Angel's soul fragments he desired. To obtain those soul fragments, they would have to rely on worlds in realspace, establishing believer worlds where troops could be stationed, more servants of Grandfather recruited, and more followers of Nurgle converted.
Mortarion dispatched the Second and Fourth Plague