Greyhawk

Greyhawk is a fantasy campaign setting for the Dungeons & Dragons role playing game originally developed by Gary Gygax as a setting for his early role playing game Chainmail, the medieval themed role playing game that later morphed into Dungeons & Dragons role playing game when merged with Dave Arneson's more fantasy oriented Role Playing Setting Blackmoor in the 1970s. The World of Greyhawk published in 1980 was the second campaign setting published by TSR; however it the setting most identified with early development of Dungeons & Dragons.

Greyhawk was initially conceived by Gary Gygax as an underground dungeon oriented adventure for his group of friends which over time expanded to become a multi-layered underground complex near the city of Greyhawk, from which the campaign setting takes its name. Gary Gygax continued expanding the world to include the lands surrounding Greyhawk and finally the continent and the fictional world of Oerth in its entirety.

Integration into Dungeons & Dragons Online
Set in Kron Hills The Temple of Elemental Evil, as a part of Update 25, brings characters and players to the World of Greyhawk for the first time.