After the release of the Nurgle Battletome and Chaos Daemons Codices near the beginning of 2018, we were blessed with a plastic Great Unclean One, who's kit was accompanied by the creation of a new special character; Rotigus the Rainfather.
Also known as the Rainmaker and the Generous One, Rotigus is a Great Unclean One most attuned to the fecundity and generosity of Nurgle and, by extension, the natural world. As such, he's seen by many primitive peoples as a fertility God. But since he's still a Great Unclean One, there's an AIDS-y twist to it. For example, someone praying for more fertile livestock will find that this does happen, but the offspring produced will be malformed diseased monstrosities, or someone praying for rain after a drought will be flooded with gross and brackish waters. He, like every Great Unclean One, is creepily happy about the jobs he does, actually humming while he infects loads of people with his diseased shitwater. He is constantly surrounded by a storm of filth and disease known as Nurgle's Deluge, so merely being near him is likely to get you drowned in sewage.
In the 8th Edition Chaos Daemons Codex, it says that, after the Tau conquest of the planet Dh'arthan, a massive drought and the failure of their hydrotech forced the Earth caste farmers to convert to the natives religion and start praying for rain... only for them to learn that their fertility god was Rotigus, who quickly transformed their desert planet into a stinking bog.
We got a little bit more information about Rotigus during the events of the War of the Spider chapter of the Psychic Awakening, as it was him who gifted Typhus the cornucopia relic thingy that Fabius Bile was after.
Later, he plays a much larger role during the third book of the Dark Imperium series, Godblight. In the third book, we see that Rotigus is one of Ku'gath's biggest rivals, with the two vying for the very special role of "Nurgle's Big Fat Favorite Boy". During the closing chapters of the Plague Wars, Rotigus indeed ends up surpassing Ku'gath as Nurgle's favored son, as it was because of him that Fabien Guelphrain, a not-Remembrancer tasked by Guilliman himself to uncover the true history of the Imperium, leading him to uncover the only book left intact in Macragge's largest library, after a battle between Rotigus and Vero Tigurius himself. Said book is an historic account of the Imperium Secundus, of all things, the truth of which Rotigus claims will ensure the decay of the Imperium from the inside out.
On the TT
Rotigus has the exact same cost and statline as a regular Great Unclean One so the comparisons are pretty straightforward. Both are psykers, with a GUO knowing two spells from the nurgle discipline and Rotigus knows one spell more and can also deny two times instead of just one. On the shooting phase department, the GUO can be equipped with a Flail for an Assault 3 AP-3/D2 that uses his own strenght, can be shot like a pistol and distributes all its damage whereas Rotigus has the "Streams of brackish filth", a decent 2d6 flamethrower with AP-3/D1 that uses his strenght that rerolls failed wounds. The Biggest difference is that the Unclean one can choose his warlord trait whereas Rotigus comes already equipped with a Pestilent miasma, an aura that causes mortal wounds on every enemy unit in melee on a +4
In the fighting department, Rotigus can slap some bitches hard with 5 S8/AP-2/W3 attacks rerolling failed wounds(both attacks and Strenght decaying as he looses wounds) and to help with hordes he gets an additional 1D6 attacks with S7/AP-1 thankls to his fanged maws and a bit some with some weak nurglings helping for an extra D6 attacks with paltry 2S.
Overall a nice guy all around, Nurgle guarantees that his boy will certainly live up any party he is thrown at with competency