At the end he fights Satan for an endless eternity, using a diminishing supply of other willing demons as spare body parts as he wears all his limbs out.
The shocks of their battle cause irreparable damage to Earth, and the last of humanity to perish with it.
At the end, he lies limbless, dead or comatose next to Satan on the moon, who asks him almost child-like for closure.
White glowing meteorites presumably sent from God crash into the Earth, wiping all demonic (and presumably all) life from it, similar to the book of revelations