Mordenkainen is a wizard from the World of Greyhawk. One of the most powerful archmagi of the Flanaess, he has played diverse roles throughout history, but is most notable for being the shadow leader of the Circle of Eight, a cabal of eight magi and a great force of neutrality.
Mordenkainen invented several spells used widely across the multiverse, including Mordenkainen's faithful hound, Mordenkainen's magnificent mansion, Mordenkainen's private sanctum, and Mordenkainen's sword.