Mordenkainen is one of the most powerful archmagi of the Flanaess. In the World of Greyhawk he has played diverse roles throughout many stories and adventures, 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.
Resources about Mordenkainen sometimes disagree with each other, since even the official publications for the World of Greyhawk sometimes contradict each other. It is clear, however, that he is one of the greater heroes (or antiheroes) of the Flanaess.
There is an article about Mordenkainen at Greyhawk Wiki.