Okay, let's sum this up.
1. The main character is immortal.
2. The main character can grow lost limbs back.
3. The main character has infinite health.
...why are there any enemies in-game? Or boss fights for that matter? Why didn't the main character kill his demon nemesis right after he was given immortality? Why did his greatest enemy even grant him a superpower that basically grants his oponent victory, sooner or later?
Flawed plot is flawed. Don't give me the "he was suffering all these years, that was the demon's idea", an omnipotent, powerthirst and immortal creature does not grant immortality, especially to its enemies.