This is a Fan Fiction of Reincarnator

In a flash, the world was gone. In its place, a new world of fantasy and power encompassed the life of Zero. For 29 years humanity fought hard against creatures and races of unimaginable might. Through the suffering, they rose strong. Yet their strength accounted for nought in the end. Billions killed as the rulers of the Abyss conquered their race. 

Yet, in the darkest of nights, a light can be found. Thus, with the support of the remainder of humanity's forces, Zero was returned to the beginning. The beginning of his journey into the Abyss, with one mission. Save humanity.