Karsus

Karsus was the most powerful mage of his time, possibly ever. He created and sustained the floating city of Eileanore. He was considered a neutral state in the Empire Wars, and worked to end the conflict. He is particularly well known for being the only person to ever develop a 12th level spell, though all that is known is that casting it led to devastation and didn't just end the wars but destroyed both the Netheril and the Raumathar empires.

These days he is mostly remembered as a warning for Mages who try to push too far, with too much hubris.