Zagyg
Zagyg Yragerne is the madman architect of the thirteen basement levels underneath Castle Greyhawk. Not so much a big bad evil guy as a big mad crazy guy.
The name is "Earnest Gary Gygax" pronounced backwards.
Gygax with Kuntz later retconned the basement of Castle Greyhawk to have 50 levels instead of just 13, and removed Zagyg as the architect. Zagyg was written back into the story as a demigod whose domains were puzzles, secrets, and practical jokes.
When Gygax was chased out of TSR by a terrible person, the intellectual property he was allowed to keep was anything based on his name, so Zagyg left TSR's games the same time Gary Gygax did. Gygax later republished his original Castle Greyhawk dungeon as "Castle Zagyg."