Riddler was a man who sought to learn the secret of death and become a god. He built the Paradox Tower that includes a library and a challenge on most floors to get to the next one. He eventually died but his ghost is still in the tower and will reward those who answer his mad questions.
Riddler may have been the man, Porgol, who was an apprentice to Arcian whom he betrayed. He sought to become a great necromancer and killed his master in trying to gain power.