Back in edan, adam and Lilith actually had a kid before lucifer showed up and Lilith left with him, the kid would be the reincarnation of the doom slayer reborn in a world without a hell and demons.
He would have peace, with his new father adam until the day it happened, his new birth mother coming back and unleashing a new hell and demons on existence, and in the first age, the first battle between this new hell and heaven, he would fight it all alone and rib and tear until it was done,
He would be captured, as his power wasn't what it was back in his first life(he would still have the power, it would just be dormant.) And slumber until he awakes thousands of years, feeling his father's life in danger and breaks out of the deepest part of hell to save him, and hell becomes a battle ground as the doom slayer returns