Sono Kojou ni Yuusha Hou Ari!

Sono Kojou ni Yuusha Hou Ari is a simulation game in which the hero Oura sets out on a mission to destroy the demon king. At one point he stumbles upon an old fortress. And within that fortress he finds an ancient super weapon. The weapon’s name is “Hero’s gun”, and it holds a strange power. It can only be operated once he had sex with a girl.

…Well? What are you waiting for!?