Karsus

The landmark event of the last age of Netheril was the birth of Karsus about 3,100 years after the nation's founding. A magically precocious child, he instinctively cast his first spell at the age of two, and at age 22 became the youngest arcanist to ever create his own floating city. Gifted with magic, but lacking the necessary discipline that comes with hard work and research, he founded a school of magic on his enclave and encouraged the attendance of radical thinkers and those interested in taboo projects. One of his students, now known only as Lord Shadow, wrote a treatise on the exploration and exploitation of other planes, such as the Plane of Shadow, which garnered initial criticism but was eventually embraced as the key to an infinite source of shadow-creatures as slave laborers as well as a dumping ground for unwanted garbage.

Shouldering the responsibility for preserving his civilization, Karsus finished creating a spell, Karsus's Avatar he had been developing for years. This spell would steal the power of a deity and transfer it to the archwizard that cast it. Karsus believed that with the power of a deity at his disposal, he could destroy the phaerimms and unite his people. Karsus cast the spell in the 3,520th year of Netheril (just over 1,700 years ago) and chose Mystryl, the goddess of magic, as his target, feeling that she was the most powerful deity and the most appropriate choice for his purposes. His spell worked all too well, and Karsus gained the powers over all magic.

Unfortunately, his choice was a terrible mistake, for one of the responsibilities of the deity of magic was to regulate the flow of magic to and from all beings, spells, and magic items in the world. Lacking the experience to do so properly, magic surged and fluctuated. With her last remaining bit of power, Mystryl sacrificed herself to block Karsus's access to the weave, causing all magic to fail. The flying cities of Netheril plummeted to the earth. The severing of the link also killed Karsus and transformed him into stone, and the last thing he saw was his entire civilization being destroyed because of his actions.

In the game Neverwinter Nights: Shadows of Undrentide. You meet Karsus in a hell created by a book in the Great Library of the risen Netherese city of Undrentide. He is being tormented by demons but you can redeem him by re-writing the magical tome in which his spirit is imprisoned.