Sonogo, Yuusha no Sugata o Mita Mono wa Inai Ver.1.1.2 by Poison - Japanese Hentai Manga

Immediately after the overthrow of the Demon King, the hero Asta turns into a girl with the help of unknown magic.
Asta loses her heroic abilities, but realizes that it is useless in a world that is starting to peacefully rebuild itself.

Aster starts a new job and is enjoying his second life to the fullest.
Until he encounters a new problem. In various places, "real demons" appear, which should have disappeared after the overthrow of the King....

This is just the beginning of an epic story that transcends time and shakes the fate of the planet.