A Great UnClean One is a grotesquely corpulent Greater-Daemon of the Plague-God Nurgle, the Chaos-God of disease, death and decay. These loathsome daemons are the harbingers of the lord of rot and ruin, and amongst Grandfather Nurgle's greatest servants, bearers of his most sacred plagues and poxes. Their forms are squat and mountainous, covered in rotting flesh and open sores that weep foul rivulets of pus. Hot ropes of intestines dribble out of huge tears in their enormous, bloated and distended bellies. This horrific girth is supported by two impossibly small and atrophied-looking legs, and their inordinately large, bulbous heads are crowned by enormous stag-like antlers.
Great UnClean Ones are facsimiles of the Chaos-God himself both physically and in spirit. Every Great UnClean One is also actually Nurgle himself in some sense, and their followers often refer to them as "Papa" or "Father Nurgle." Great Unclean Ones are seldom deathlike or morbid in character; in fact, they are usually motivated by the same trivial enthusiasms that drive the living. They are gregarious and even sentimental in their nature with a remarkable fondness for their followers. They often refer to their followers as their "children" and take great pride in their appearance and oddly endearing behaviour. Indeed, it is not uncommon for Great Unclean Ones to compete amongst themselves in the matter of spreading Nurgle's plaguesome blessings across the galaxy. They take great pride in the achievements of their fellow servants of Nurgle, proclaiming vociferously the splendours of the poxes and sores evidenced by those around them, and laugh heartily at the death and destruction wrought in Nurgle's name.
Their appearances on the mortal plane is heralded with an ear-shattering howl of glee, as they burst through the veil of reality, delighted to walk upon the firmament of realspace once again, spreading their bounty of plagues upon mortals. Often disease and desolation on a planetary scale herald their arrival as entire populations are stricken with the corruption of the Plague Lord's favoured plague which is known as Nurgle's Rot. This disease is the most contagious, rapacious, and most heinous of all sicknesses, poxes, and fevers Father Nurgle has ever conceived. The afflicted mortals slowly suffer as the plague ravages their bodies, turning them into bloated, rotting, living corpses before they succumb to the inevitable, agonising demise. The bile-slick Great UnClean One then waddles through the collapsed cities and the mounds of the dying, happily bestowing his blessings, borne aloft on a living tide of chortling, chattering Nurglings.
In battle, Great UnClean Ones wield enormous rusty blades and maces of iron, crude, corroded cleavers and immense Plague Flails, each one dripping with pestilence. If threatened with bodily harm, the thousands of diminutive Nurglings that follow in the Greater Daemon's wake will attack an unwary opponent, dragging them down in a murderous wave the instant their "father" is attacked. Natural-borne carrion creatures, numbering in the thousands, gather around the Greater Daemon to protect it and trail in its wake. Hordes of sky-blackening flies, rats, vultures, crows, and worms inevitably follow a Great Unclean One's passing.
Great UnClean Ones long ago became inured to pain, a result of their abundant infections, plagues, and virulent sores. Their truly massive size and their ability to withstand almost any level of injury and pain make them dangerous adversaries, as they can absorb astounding amounts of damage before finally falling to the attacks of their foes. These daemon lords possess a hellish prowess which seems at odds with their bulk and squat stature. They barrel into their enemies, laughing boisterously, all the while laying about them with colossal rusty cleavers and seven-headed Plague Flails. The thick cloud of flies which feast at their open sores disrupt the attacks of their enemies and spread the putrid diseases of their host. All the while, the cooing Nurglings form a hideous vanguard for their patriarch and in a playful gesture do all they can to help spread their progenitor's beloved plagues to all those nearby.