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.
In 8th he's still propa tanky. He has retained his toughness of 9, which isn't as good as it used to be, but he now boasts 22 Wounds, almost as many as a knight, bit less than his Khorne or Tzeentch counter-parts. He has disgustinly resilient on top of his 4++, so his FnP and save have gotten slightly worse. His shrouded rule has been replaced with a -1 to hit penalty to to melee attacks that target friendly "Nurgle and Daemon units" within 6". Which RAW gives him synergy with the Daemons of others Gods, Plague Daddy's generosity truly knows no end. But since he lets friendly Nurgle units use his leadership, its still probably better to use with other Nurgle units. His other stats are typical for an Arch Daemon, WS and BS are 2+, Strength starts at 10, Movement at 7" (This pleases Nurgle) 6 attacks and he has a healthy leadership of 10. Like the other Arch Daemons, he had Arch-Daemon ritual, letting you roll 9 die to reach 30 powers and summon him. Also like other Arch Daemons, he had degrading stats, specifically his movement, strength and attacks. For weapons, he's lost stomp, but he gained Horrific Vomit, a 2D6, 9" Pistol that auto hits at strength 6 -2AP and D3 damage. So he is easier to tarpit with cultists and conscripts, but harder to tarpit with Space Marines. In melee he still has the Blade of Decay which is S+2 -4AP and does 6 damage. The Nurglings infesting him also get to make D6 attacks at strength 2, but they get to re-roll wounds, how nice. On top of this, he is a Psyker that can manifest and deny 3 powers. He knows smite and 2 powers from the Nurgle Daemon's list. Ultimately, he just Greater, More Unclean Big One, if that's a bad thing.