So during the Reichenbach Falls, instead of faking his death, Sherlock made a deal with a demon (maybe Moriarty??) for his soul in exchange for the permanent safety of his friends. Instead of ten years Sherlock is given ten minutes (makes the phone call to John, prepares himself for death) and makes it look like a suicide to keep John from wondering what happened to him. He is take to hell and tortured for decades until he is mysteriously rescued. In the real world it's only been about 3 years.

Modify however you like I just really want to see Sherlock in hell being tortured and then rescued.

Bonus points if John had no idea what actually happened to Sherlock and tries to punch him before he notices how different he looks from 3 years ago