The Demon King's Immortal Vessel

Fate was a normal child living with his mother when he encountered the dying Demon King. Fast forward to a couple of years later, he is immortal and in prison. He spends his waking hours wondering how he would be killed each day. 

One day nearly eleven years after the supposed death of the Demon King, a woman suddenly appears in his cell and promises to break him out. Of course, he doesn't believe her, they were in the inescapable dungeons of Euthania. She couldn't break him out of these dungeons, let alone, all by herself. It was impossible.

Somehow she succeeds but that's just the beginning of his problems as everybody wants a piece of him. How long before he is caught and returned to the prison or would he find a haven or better still, awaken the powers of the Demon King and protect himself?