Greyhawk was a dungeon under a castle created by Gary Gygax to entertain his children and friends but quickly grew into an entire world as their adventures grew. So popular was Greyhawk that over twenty people gathered each night to play in Gygax's basement. At first, Greyhawk was just one level, but was expanded to thirteen. Later on with the help of Rob Kuntz, the world grew to over 55 levels when the campaign ended. Greyhawk became one of the first official campaign settings of the role-playing game Dungeons & Dragons, paving the way for later universes such as the Forgotten Realms.
In 1971, within a Minneapolis war-gaming club, a man called David Wesely grew tired of his group's Napoleonic setting. He placed a castle figure where the battlefield was supposed to be and said told his players to select a soldier and descend into the Barony of Blackmoor's perilous castle dungeons and explore their depths. When Gygax heard about this new role-playing game, he traveled from St. Paul to Lake Geneva to test it out, and knew that this was going to be huge.
From that meeting, not only was the world of Greyhawk born, but Dungeons & Dragons itself was formed.
During the first rounds of Greyhawk, players didn't have any specific deities to call out to aid them. Some of them had to take matters in their own hands and summoned strength from the old Norse gods of Thor and Odin. To rectify this, Gygax jokingly created the deity Saint Cuthbert of the cudgel, who persuaded non-believers into worshiping him with his mighty cudgel. He also created Pholtus, whose believers are so fanatical that they refuse to believe that any other gods besides Pholtus exist.
In Video Games
One of the earliest and most challenging Greyhawk adventure modules, The Temple of Elemental Evil, was adapted into a PC RPG by Troika. In this adventure, the player must guide a party into the namesake temple and thwart the ancient evil residing within. The game is notable for its strict adherence to the Dungeons & Dragons 3.5 Edition rules that guide every aspect of play.