The Forbidden Fountain, as seen on the cover of The Forbidden Fountain of Oz.
The Forbidden Fountain contains the Water of Oblivion, the "most dangerous" thing in Oz. Whoever drinks suffers instant amnesia.
Description
The Fountain stands on the grounds of the Royal Palace in the Emerald City. Clear, sparkling water bubbles up in its golden basin, but it bears a sign that reads, "All Persons are Forbidden to Drink at this Fountain."
History
Centuries ago, a wicked king ruled the Land of Oz and made himself and his people miserable and unhappy. Glinda placed the Fountain in its present location, and the king drank from its water. His people still feared him, so he made them drink from the fountain as well. Then the king and his people grew wise together. That king placed the warning sign on the Fountain to prevent anyone from drinking of it by accident.
Ozma used the Water of Oblivion to defeat the invasion of the Nomes, Phanfasms, Whimsies, and Growleywogs. (The Emerald City of Oz)
Later, the Nome King and Kiki Aru were made to drink from the Water of Oblivion after attempting the conquer the Land of Oz. (The Magic of Oz)
Background
Some later Oz writers have found the Fountain irresistibly tempting. It is the crucial element in the McGraws' The Forbidden Fountain of Oz. It also plays important roles in Jack Snow's The Shaggy Man of Oz, Rachel Cosgrove Payes's The Wicked Witch of Oz, and Edward Einhorn's Paradox in Oz.