With the world at peace, it is an era where conquering dungeons has been transformed into entertainment. Adventurers and monsters are able to fight with a substitute body made of magic, popularizing dungeon conquests with no deaths. In a continent where dungeon conquests are live streamed, the party I was associated with was supposed to have been the 4th most popular in world. However, I was told that I was hindering them from climbing to 3rd place. I struggled with finding another party until one day a beautiful woman appeared before me telling she had a job for me. Impressed that her abilities were at a value higher than my former comrades, I ended up taking the interview before hearing what the job entailed. In deepest part of the dungeon with the highest difficulty, the demon lord's castle, one of the Four Devas was waiting for me. This is the story of the black mage who was expelled from the heroes party as he enters into the demon lord's army and repels the heroes.