The Suel Imperium was the original empire of the human ethnicity known as the Suloise on the Dungeons & Dragons world of Greyhawk. They were decadent and cruel: think, Melnibonéans with a touch of elfaboo. Accordingly, the Imperium subjugated all non-human races and non-Suloise humans it could get its hands on. This worked fine, for them, until they conflicted with their neighbours to the north, the "Baklunish" Arabian expies here.
A cold war ensued that went hot. Very hot, once the Suel sicced the hordlings upon the north and the Baklunis rained the Rain of Colorless Fire upon the south. This latter reduced the Suel heartland to a barren waste called the Sea of Dust, and scattered the survivors far and wide.
The Scarlet Brotherhood is that offshoot of the Imperium which has, in the finest Bourbon tradition, retained the most and learnt the least. They were "old men of the mountain" in 1e, and hosted their coming-out party in the Greyhawk Wars. One thing they did figure out is that brute power is a Bad Idea, especially since all Suel are a minority now; so they go more in for subterfuge.
It's been noted that Suel resemble Nazi Aryan propaganda: pale skin, violet/blue/gray eyes, blonde or red hair. Although, the worst Suel are lighter on those beefy muscles Ernst Roehm was into, the stronger northern barbarians being not so into pan-Suel ideology.