Rhea, With my demon king's system

Leo, a lifelong servant of humanity, finally defeats his arch-nemesis, the demon king. In a twist of fate, the demon king bestows a gift upon him, revealing the bitter truth: humanity's betrayal and manipulation of Leo throughout his life. Shocked by this revelation, Leo questions the demon king's motives, who seeks peace and sacrifices himself for it.
Seven hundred years later, Leo is reborn as a half-demon. The world has changed dramatically: humanity, once the dominant force, is now weak and divided, while the demons have risen in power. The elves, humanity's former allies, have sided with the demons. Leo's once-glorious empire, a symbol of human power, is now reduced to a mere footnote in history, its legacy tarnished by betrayal and corruption.
Despite the harsh realities, Leo remains committed to saving humanity. He forms deep bonds with his demon companions, experiencing love and loyalty that he never found among humans. However, a new threat emerges: a group of self-proclaimed gods who wage a devastating war, decimating the human and demon populations alike. Leo, armed with the power of the demon king system and the enigmatic Mother of Gods, must now confront this new challenge and decide the fate of all beings.