Easy. Normal. Hard. Unreal.
One morning, time stops, and a voice calling itself the Enigma asks every human on Earth to choose a difficulty. The harder the setting, the greater the reward, and the lower the odds of coming back alive.
Jason Keller, an ordinary engineering student, chooses Normal. The Enigma refuses. For him, there is only one setting: Calamity.
"You must endure the calamity to become the calamity."
Mission after mission, the Enigma throws Jason into other worlds and into trials no one was ever meant to survive: an endless ocean, intergalactic warships, the basement of an eldritch horror. Each one takes something from him. Each one leaves something behind: a skill, a scar, a little less of the student he used to be.
His first trial is a slow-burn battle against solitude and despair. The next will be something else entirely, and so will the one after that. Every calamity presses him a little further into shape, until he fits the perfect mold the Enigma has made for him.
The other difficulties have an end. His doesn't seem to.