In the world of
Greyhawk, Kyuss was a very powerful
Flannish
necromancer (
neutral evil,
human) who sought
deification. He has at least achieved the Greyhawk qualification of "
Hero-Deity" (of the creation of undead) which corresponds to those who are but a half-step towards full fledged deifications. As of the date of the "current"
Living Greyhawk chronicles (that would be CY 592, which corresponds to publishications made in 2002), it is unknown as to whether or not he has fully ascended, but it is quite likely that he has. (He shows up as a valid deity for NPCs to worship in the LG campaign.)
Amongst those who are not familiar with Greyhawk, Kyuss would be most famous for his undead creations. "The Sons of Kyuss" (appearing in the Fiend Folio) look like zombies with maggots crawling in and out of their eye sockets. His other undead creations are just as gruesome. (My favorite is the gargantuan undead maggot. How odd.) His creations fill the area where he was last known to live, "The Wormcrawl Fissure."
The "modern" info about Kyuss is detailed in Living Greyhawk Journals 1-3, Dragon Magazine 270 and Dragon 276. There's also information about him in Rary the Traitor. There's some activity in the Living Greyhawk campaign going on in the County of Urnst involving either worshippers of Kyuss or Kyuss himself. (I'd be very interested in finding out, but Living Greyhawk only allows you to play in modules which correspond to your real life location. I happen to live in Furyondy -- Michigan -- which is a distance from where the location that CoU corresponds to.)
Although Vecna is the most famous villain from Greyhawk, Kyuss is just as evil and predates him by almost a century.