Tou no Shita no Exercitus by Astronauts: Comet - Adult Sex Game

A long, long time ago, a certain civilization lived in prosperity with its superior technology until it destroyed itself due to over-development. The world was destroyed in this "accident", but it gradually recovered after a long time. However, it was limited only to the areas surrounding the Tower of Velvalem, built by the "family of the dawn".

Then a new problem soon arose. The number of people under the protection of the "tower" had increased too much, causing its circulation ability to become unstable. After plenty of hand-wringing, the leaders at that time created a manic system with "demons" coming out of the tower to attack and reduce the human population, and consequently preserve the balance of the world.

A hundred years later, Melcriti Academy was founded as a training school for adventurers who fight against these demons. They were able to repel them and the population began to increase once more. As a result, Velvalem became unstable and the countdown towards destruction is underway. Yuuki was accidentally summoned by Chloe into this world. He must fight as an adventurer through various challenges along with the heroines.