Story
The demon queen subdued enormous and powerful monsters and slaughtered over half of the world's population. Surviving members of the human race elected a hero to slay the demon queen, beseeching release from her oppressive rule. However, he questions his mission to eradicate the monsters in his opinion they too are conscious and alive, and so he anguishes over that fact. Sheila the Demon Queen became interested in him and joined his journey, hiding her identity with the plan to kill him after she has gained his trust.
Synopsis via AniList (English).