Ossan, Oniyome e no Fukushuu o Chikau

Seventy years ago, dungeons suddenly opened up across the world. Inside them, [Mana] was discovered — a substance that dramatically accelerates human evolution. One day, the protagonist Seishirou rescues a woman inside a dungeon. That woman turns out to be an “Oni” — a special-class being embedded with high-tier mana. Before long, the two of them become a bridge connecting “humans” and “Oni” together. …Little did he know, at that time, that this very woman would one day betray him and take everything from him. Original Webtoon