A deep and dark cave located inside a dome in Japan.

The endless corridors boasted a depth that seemed capable of swallowing even light.

The humid air seeped deep into the lungs, and droplets of water constantly fell from the moss-covered walls, breaking the silence.

The uneven floor and slippery mud made each step slow.

The winding passages were so tangled that they completely robbed one of their sense of direction.

Through this, a group of people silently moved forward.

At the front of the group was, surprisingly, a young girl.

In her hand, she held a black sword, its …