The Barbarian Hordes the the north, the Fruzti and the Hold of the Stone Fist and others are also Suel, so they get alot of breeding stock and members from there.

They do have a power base, the Tilvanot Peninsula, separated from the rest of the Flanaess by the Vast Swamp. They have enslaved all the people of Hempmonaland giving them an army of Aztecish warriors, and their breeding program extends to the humanoid scum they have also enslaved.

Their divine patron is Tharizdun, and they have several other gods, Pyremius, Beltar, Syrul and Llerg as well as Wastri. Oh, and Wee Jas. They are also the ones who blew up the Baklunish Empire with the Invoked Devastation.

So they have a base of operations thats nearly impossible to invade, hordes of violent people that they like and who are willing to fight and breed for them, fanatically loyal members desperate to move up in the ranks, and several armies worth of less technologically and magically advanced slaves, along with a handful of gods in their corner, and they serve the greatest most powerful evil that Oerth has ever seen.