Whereas most Greater Daemons are actually servants and slaves of their Power, those of Nurgle are more akin to facsimiles of Nurgle himself both in terms of appearance and personality. Indeed the followers of Nurgle frequently refer to Great Unclean Ones as Father Nurgle - which gives an indication of the relation between Daemon and underling. Nurgle's followers are treated with a gregarious and even sentimental nature by Great Unclean Ones, who seem to take a noticeable pride in their behavior and achievements. All Greater Daemons of Nurgle seem to have boundless energy and drive, constantly working to extend the process of rot and decay, thoughtless of teir own comfort while places still remain uncorrupted.
Greater Daemons of Nurgle resemble Nurgle himself in appearance and personality. Their bloated bodies ooze with sores and boils, pus and slime dribbles over their leprous skin, and decaying inner organs protrude from tears in their great bellies. Great Unclean Ones treat their followers with a gregarious and even sentimental nature, taking noticeable pride in their slaves' behaviour, diseases and achievements. All Great Unclean Ones seem to have boundless energy and drive, constantly working to extend the process of rot and decay, thoughtless for their own comfort while parts of the galaxy still remain uncorrupted. Taken from Renegades
A Great Unclean One resembles Nurgle himself: huge, green-skinned and bloated with disease. From sores and boils, pus oozes over the Daemon's body, glistening on leprous skin. Its body swarms with tiny Daemon Nurglings, which pick at its skin, squabbling for their master's attention. Taken from WD 230