Based on the adult manga by Sawaru Erect. The third series from the Pandra franchise.

There once was a demon Astaroth who invaded the human kingdom, plunging the world into war. However, he was finally suppressed by a group of heroes who took his powers and sealed him deep underground. Two hundred years later, there were rumors that the seal was broken due to the recent shifts in the Earth’s crust and that the demon had been revived. Many confident adventurers descended on Astaroth’s underground labyrinth with the intention of defeating him and becoming famous. Little did they know that he had set out a trap for them…