''Rrrbit! Rrrbit! Great Jove has found you unworthy, human! Now prepare to die !''
100 Word Society
These spellcasters are the few who survived the great trials to become a being of both arcane and divine magic.
Believable magic, like ordinary physicis, operates according to some invariable laws that always result in some kind of cost or "bounce back". The grater the magic, the more it should cost the character physically, emotionally, and spiritually.