This is a series. Set in the Marvel Cinematic Universe universe.
A surgeon destroys his hands in a car crash, spends everything he has looking for a cure, and ends up in Kathmandu learning sorcery from people who were not offering one. He steals from a book he was told not to open, and beats the thing coming for the Earth by trapping it in a time loop and being willing to die in it over and over until it agrees to leave.