The Plague Planet is the Daemon World of the Daemon Prince Mortarion, and homeworld of the Death Guard Traitor Legion. It is located within the Eye of Terror and is a world where disease is the norm and the viridian miasmic clouds that shroud the world bring contagion and death. Its diseased inhabitants pray to Nurgle for relief from their constant agony. Some of them are favoured and become his Chaos Champions, and then fight among themselves for mastery and the chance to become Daemon Princes themselves. Beastmen make up a large part of the world's population; they are bloated with disease and flies swarm around them and breed upon their rotting flesh. The Beastmen are very proud of these signs of their patron God's favour, and openly boast of the number and severity of their diseases. The Plague Marines of the ruling Death Guard involve themselves in carrying out Mortarion's wishes and travelling to other worlds to spread Nurgle's plagues. All spacecraft which succumb to plague are eventually drawn, through the Warp, to the Plague Planet, where they become part of the planet's Plague Fleets. These Plague Fleets carry followers of Nurgle through the Warp to inhabited worlds where their destructive raids are inevitably followed by outbreaks of deathly contagion.
Mortarion has shaped the planet to resemble his former homeworld of Barbarus. Its citizens cower in tiny villages, serving their supreme masters of the Death Guard, who reside in mighty fortress citadels far above all of them. There are creatures that should be dead and are diseased far beyond reckoning walking the planet. All is ruled by the skeletal Mortarion, enthroned upon the highest part of the planet, much like his foster father was on Barbarus. Because the Plague Planet is located close to the fabric of reality in the Eye of Terror, it is an excellent place for the Forces of Chaos to launch raids and invasions from, especially against Imperial worlds in the Segmentum Obscurus.
- Crusade of Fire (Campaign Book), pg. 85
- Realm of Chaos: The Lost and the Damned, pg. 257