Astro King: Shoukan Yuusha dakedo Kakyuu Ninteisareta node Maid Harem wo Tsukurimasu!

Called to another world as heroes, a group of high-school students were summoned to clear a dungeon. They were lured in with the promise of unlimited sex with the maids of the castle. While his classmates were being defeated by the dungeon one after another, Yoshitsune, who was originally summoned with a low-rank, used his cheat skill to steadily increased the number of maids in his harem...?

(Source: Comic Valkyrie, translated)