Scabeiathrax

Scabeiathrax the Bloated is the most powerful of the Great Unclean Ones of Nurgle and the second most disgusting thing in the Warp or outside of it. He has only been summoned twice in the last thousand years (thankfully for those in realspace) due to the fact that only Nurgle's most devoted cultists have the persistence to summon him. These two instances were the Contagion of Viriath and the Siege of Vraks itself, and in neither case did he stay long, preferring to chill out at his plague planet, the Blighted Pit.

Despite his loathsome appearance, Scabeiathrax is a happy guy (like most of Nurgle's followers), even though his mere presence causes living things to rot away into toxic sludge. On Vraks he did just that to the Ordo Malleus Inquisitor Vokes before getting banished by Captain Stern of the Grey Knights.

On the Tabletop

Nurgle's representative for the uber-daemons club, a Daemons Lord of War choice clocking in at 777 points. He can only be used in armies of 2000 or more points, leaving him mostly the domain of Apocalypse games. Sporting a ridiculous toughness of 9 (making him immune to S5 and below), 6 wounds, FNP(4+), a 3++, and Shrouded due to being a Daemon of Nurgle, he's damn near impossible to kill. His weapon is the Blade of Decay, which makes every wound he inflicts become 2, and anything within 6" of him that doesn't have Mark of Nurgle or Daemon of Nurgle has to take a toughness test or lose a wound with no armor or cover saves allowed. Between this and Stomp for being a Gargantuan Creature, he's almost impossible to tarpit.