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 its foetid globe bring contagion and death.
A World of Death
When the Warmaster Horus' rebellion collapsed, the Death Guard fell back to the Eye of Terror in good order, reaving as they went. There, Mortarion earned his ultimate reward from Nurgle: daemonhood, and a world to shape and rule as he saw fit.
The so-called Plague Planet he made is a dark reflection of an already shadowed world. It is Barbarus, Mortarion's original homeworld, remade in yet more hideous hues, a fume-wreathed nether-hell where disease runs rampant and barbarian tribes cling to a nightmarish existence amidst rotting forests and swamps. Worm-eaten mountains rise to pierce the churning skies, their flanks roamed by plague-ridden abominations that should be dead yet still prey on human flesh. Higher, atop the peaks, the Death Guard maintain their fortresses as once the carrion lords of Barbarus ruled from on high, revelling in the bitter irony that they have become the very despots they once fought to depose.
The clouds of contagion that wreathe these slab-sided, rusting fastnesses are inimical to life. None but the Death Guard can endure even a single breath of them. Quite besides the brutish batteries of orbital guns that stud every single mountaintop of this world, its vapours alone have proved enough to massacre invading armies on more than one occasion.
The Plague Planet is replete with hideous spectacles and places of foetid power. Amongst the suppurating valleys rise tri-lobed platforms of rusted iron upon which mortal tribal champions battle for the right to join the Death Guard's ranks. Hanging in orbit are chained rings of biomechanical defence platforms and suppurating orbital docks, at which the Plague Fleets suckle like flies on a carcass. Shrines to Nurgle, crawling molluscoid fortresses, smog-spewing plague factories and crackling alchemical shield generators dot the planet's surface. The world's mortal citizens cower in tiny villages, serving their supreme masters of the Death Guard, who reside in mighty fortress citadels far above all of them.
Greatest of all, though, are the seven mountaintop keeps of the Death Guard's Plague Companies, one company ruling over each in their own uniquely revolting fashions. The largest of these, known as the Black Manse, stretches between the three highest peaks of the Plague Planet and serves as Mortarion's personal keep. The dungeons and laboratories of this nightmarish fastness conceal horrors of the very darkest kind.
The diseased mortal inhabitants of the Plague Planet 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 also 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 voidcraft in the galaxy 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.
In the end, all is ruled by the skeletal Mortarion, enthroned upon the highest part of the planet in the Black Manse, 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.
The Books of Woe
Within the mouldering libraries of the Black Manse, Mortarion hoards endless treatises and forbidden tomes of numerology, cryptoscience, abhorrent scripture and arcane conjuring. The foulest of these are the seven Books of Woe, aberrations whose covers drip with filth and whose pages moan like a bloated corpse when turned. On occasion, Mortarion bestows one of these abhorrent tomes upon a favoured champion so that they may call upon the incantations within.
By reading from a Book of Woe, a worthy champion may tear aside the veil of reality and summon the Plague God's daemon legions into reality. They can draw the Garden of Nurgle itself into realspace, and -- during cursed stellar conjunctions -- even transform planets into befouled hellscapes, worlds of Nurgle manifesting in reality like sores upon a sick man's body.
The Peal Yard
Deep within the baleswamps of the Plague Planet lies the Peal Yard. This fortress-factory rings with a constant cacophony, for it is here that the bells known as the tocsins of misery used by the Death Guard's Noxious Blightbringers are forged.
The Peal Yard is an interstice, existing upon the very cusp of the Warp. Within, droning daemon smiths cast the cursed bells from the screams of dying psykers, beating runes of madness and misery into them under the watchful eyes of Lord Gothax's Tollguard.
The disease factories of the Plague Planet are as bloated and corrupted as the beings that wander their halls.They are sprawling abominations of gurgling pipes, bubbling vats, belching smokestacks and rusted cages crammed with test subjects and living ingredients. So vast are they that their masters, the Foul Blightspawn and Biologus Putrifiers, have divided them up into virtual nations, each ruled by the Blightspawn elders that fester at their hearts.
Millions-strong mortal Chaos Cults reside in these factories, scurrying to do the bidding of their foul masters and worshipping the Plague Marines as demigods of Nurgle, even as they slowly sicken and die. Devoted to brewing the plagues favoured by their ruling elders, these rag-clad lunatics regularly engage in sectarian warfare through the smog-wreathed passages and brewing chambers, much to the amusement of their Death Guard masters.
- Codex Heretic Astartes - Death Guard (8th Edition), pp. 10, 32, 34-35, 48, 51, 54
- Crusade of Fire (Campaign Book), pg. 85
- Realm of Chaos: The Lost and the Damned, pg. 257