The Great Unclean One is the greater daemon of Nurgle, a vast and jovial mountain of rotting flesh that embodies the Plague-Father's boundless, doting love of decay. Its swollen belly splits to spill glistening innards and swarming Nurglings; its hide weeps corruption; and clouds of daemon-flies drone about it in an endless, buzzing hymn. For all its horror it radiates a grotesque good cheer, greeting the slaughter with a fond chuckle, as though every death were a cherished gift lovingly given.
Despite its ponderous bulk it is appallingly hard to destroy, absorbing cannon-shot and blade alike into its rolls of putrid flesh and coming on undeterred, spreading disease with every wallowing step. It bears a great rusted blade and a flail of corroded iron, and it delights in gathering the enemy close to share its master's gifts. A Great Unclean One is decay made vast and merry — the terrible truth that all things rot, wrapped in a fly-blown smile and shambling forward to embrace the world.