Also known as Plague Lords, Decayed Ones, Fly Masters, Stench Lords, Father Nurgle
The servants of Nurgle wander the Chaos Wastes in great cavalcades. Ever-marching, they move from place to place, expounding on the glories of pestilence, imbuing all things with sudden tumescence, leaving in their wake fields of blistering, swollen, split, and rotting corpses and vegetation. Plaguebearers count the glories of their hideous master, while carpets of Nurglings mewl and whine, picking and twitching in the perpetual becoming that defines their existence. And behind the great parade are the droves of mortal followers, driven mad with despair and titillated by the wonders revealed to them by the servants of Nurgle. But of all of the members of this unruly host, none are quite so horrific as the Great Unclean Ones.
The massive terrors are a blend of sheer grotesqueness and rampant decay. They are sinister things, whose affection and corruption evoke shuddering madness from those who witness it. They have no shame, no sense of decency, revelling in the basest acts. Fat wormy fingers probe the depths of their dripping nostrils. They randomly spray their leavings in great clouds of such pestilential power that the faintest whiff kills. They engorge themselves on food and drinking, spilling their tainted meals out onto the ground through the rents and tears in their prodigious bulk. When not indulging their bestial appetites, they laugh and pinch, coddle and coo their mewling broods of Nurglings.
On the battlefield, the Great Unclean One waddles into the thickest of the fray, muttering such foul jokes that the souls of those who hear them shrivel. It joyously slaughters foes, chortling with disgusting mirth. It might snatch a fleeing warrior to stuff the unfortunate soul into its suppurating maw only to spray the mostly dissolved carcass in a stream of noxious vomit. In the bodies of its victims bloom the most unsightly monstrosities, and the Fly Master, always patient and endearing towards its spawn, will pause to aid in the birthing of some new unspeakable enemy or harvest the shoots of fungus and corruption from the twitching dead.
Manifestations
The signs of a Great Unclean One are visible to all, even to those that would deny such a thing could ever stain the world. The air grows heavy and humid, and the very winds seem to slow, laden as they are with the faintest hint of rot, blending the odours of a rotten tooth with the carcass of an animal left too long in the sun. In the folds of clothing, buried in the smallclothes beneath armour, in the crevices of the flesh, there is tickling movement as a new maggot is born. Swollen black flies seem to come from nowhere, first individually, then in great swarms.
As the Daemon draws closer, the infestation of flies and maggots spread. They infest everything, from food to water. Fruits and vegetables ripen on the vines with such speed, that they burst, spilling their seeds onto the ground in a syrupy mess of pale slime. Everything blooms and ripens to the perfect moment of harvesting only to collapse into foul-smelling rot and perversion.
And then, the Great Unclean One appears. Its awfulness spreads throughout the land, causing lesions to appear on the flesh and existing cancers, warts, and buboes to grow. All that lives gives way to decay and rot, sagging on the vine. Clouds of flies blot out the sun, and the very air dies, filled with the stink of death. And the booming laughter and profane muttering drive those who hear it mad with despair.
Appearance
The Greater Daemons of Nurgle are among the most horrific Daemons known to mortals. Beneath clouds of swarming fly souls, they are almost perfect replicas of Nurgle himself: bulbous figures bloated with corruption. Their sickly green skin is little more than a few smooth places between mountainous boils, cavernous wounds from which spill torrents of chunky pus, and fields of quivering blisters. It is a breeding ground for every pox and blight ever to torment the good people of the mortal world.
Through the various rents of decayed and ruptured flesh, the internal organs are in full view, pumping corruption throughout its massive bodies, leaking filth and bile over its thick hide, painting it with fresh new contagions to tickle the flesh. And crawling and nuzzling and suckling and hiding on this grotesque thing are swarms of tiny Nurglings, whom the Fly Master sees as his brood. Truly, the Great Unclean Ones represent the inevitable decay and decline of all things.
Despite their macabre appearance, the Decayed Ones have a cheerful disposition, favouring their pets and followers with almost fatherly attention. Between the rancid coos and the constant praises that stream from its seeping maw, its existence is maddening. Great Unclean Ones take great pride in the achievements of their fellow creatures, loudly celebrating each new affliction, new pox, and new sore with exuberance. When faced with the destruction wrought in Nurgle’s name, their booming laughter can be heard for miles.
Such love of Nurgle and his children breeds a cheerful readiness to fight in his name, for the corpse-strewn battlefield is a fertile garden for new disease and pestilence. The Great Unclean One sweeps through war at the forefront of a tide of filth and decay, swinging its great flail and splashing all with droplets of corruption. The Decayed Ones butcher their way through armies with each swing of their mighty arms, as unstoppable as the inevitable march of decline.
Capabilities
The appearance of a Great Unclean One is more than sufficient to unman even the boldest hero. The air sours with the presence, filling it with a pestilential mist that causes the flesh to rebel, infesting all mortals with horrid illnesses that wreak havoc with the body and mind. With each new blast of flatulence, some new plague is born, each rumbling belch produces legions of virulent diseases, and those unfortunate to face these Daemons have little hope of living through the encounter.
The Great Unclean One contaminates the earth as much as it does the air. Wherever it goes, it leaves a trail of slippery slime that breeds Nurglings by the thousands. The grass brightens and grows until it can no longer bear the weight of its expanded form. The rocks dissolve, and the earth turns into a soupy morass of excrement and mud. What’s worse is that the trail of a Stench Lord remains for decades, polluting the earth and seeding the land with future calamities for the unwary.
Though it is awful to behold and experience, it is far worse to fight. Its dripping hide corrodes the strongest steel, and plumes of acrid smoke and droplets of acidic excreta surround the Daemon as it wades into the thickest knots of its enemies. And through it all, it disgorges the contents of its vast and often-exposed gullet, spewing an unholy combination of entrails, partly digested flesh, wriggling maggots and its own vile excrement in a torrent that not only burns but afflicts its victims with the most virulent of all plagues to stalk the lands of men.
Source
Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 244-245)