The immortal dungeon master: A low-ranking adventurer who was betrayed and abandoned vows revenge on his former female adventurer companions!

"Deep Emerald" is an all-female A-rank adventurer party possessing both beauty and skill. Galen, who aspired to be an adventurer but has no magical power whatsoever and is called a "magic-less" adventurer, lives a low-ranking life as an adventurer. To pay off her deceased best friend's debts, she takes on the role of errand boy and vanguard for "Deep Emerald." However, both her best friend's death and the enormous debt that is beyond her means were all part of a scheme. After learning the truth, Galen is abandoned in the dungeon, falls to the deepest part, and nearly loses her life, but in her dying moments, she unexpectedly receives the power of the monster's master and becomes the ruler of the labyrinth, the Dungeon Master.