The Demon Emperor

Said to once have conquered half of The Shattered Continent in the time before the continent was sundered. The Demon Lord was said to be able to destroy great cities by his own power and bend even the most good hearted soul to his cause. His goals true goals are lost to time but it is known he wanted all of Aphirion and would've done anything in his power to take it.
Some legends say he was killed in the clash between Tiamat and Bahamut. Others say he never died, that he holds up in secret to recover from a grievous wound taken in the final battle with the Shining Men and waits to return to Aphirion with his full strength to conquer once more. Another legend says he was defeated by the King of the Shining men and sealed deep below the earth so that no one could ever find him. The truth is lost to time.

Historical Basis

The Truth, as uncovered by the Balefire Venture, was that the Demon Emperor was in fact the Demon Prince Fraz-Urb'Luu. Fraz was at some point sealed beneath the country of Vandor, somewhere deep in the Underdark. He was sealed in a stone and his staff was shattered into three separate pieces, one of which was a Staff of Beguiling once held by Sprocket and stolen by a cultist. The other two pieces whereabouts are still unknown. Fraz was also being worshipped as the "God of Wishes", a belief that was being spread by an organization known as the Elemental Convocation. They had planned to build Fraz-Urb'Luu's strength before siphoning it off for themselves toward their own ends.