Based on the adult game by Astronauts: Sirius.

A demon of the Astaroth invaded the human kingdom, dragging the world into war. However, he is ultimately repressed by the unity of the hero who takes his power and seals him deep underground. Two hundred years later, there are rumors that the seal is broken because the latest in the earth's crust and his demons have been revived. Many confident adventurers descend on Astaroth's underground labyrinth with the intention of defeating him and becoming famous. Little did they know that Astaroth had made a trap for them ...

(Source: Nekopoi and Has Been Translated)