Great Unclean Ones are the Greater Daemons of Nurgle, Chaos God of death and disease.
Great Unclean Ones are often considered extensions of Nurgle himself, having been known to mimic his appearance and even personality. They are colossal hulks of rotting flesh infested with effectively every disease and sickness in existence, including those which should not exist in the material universe. In battle, Great Unclean Ones carry enormous Bileswords or Great Plague Flails, and can produce a limitless supply of Nurglings from their own flesh. Great Unclean Ones are so bloated with infectious pathogens, the mere presence of one in realspace is enough to trigger a deadly interstellar pandemic.
Despite the extreme danger of their very existence, Great Unclean Ones are known to be surprisingly jovial and friendly, particularly towards other servants of Nurgle. They often demonstrate an almost paternal affection towards their underlings and consider the rot and sickness they spread the benevolent gifts of Nurgle shared with all.