The Deep School, the Dark Academy. The one place you can go to learn the true nature of reality. Most people fail the course. They die, or go mad, or both.
However, a few extraordinary souls do make it through the course and return to the world above disturbingly powerful and strangell transformed.
People who have failed
Sigismund, the Mad Mathemagician
People who have passed
Hadleigh Oblivion, the Detective Inspectre