It's a fantasy setting, not an isekai. The entire setting takes place in one singular world, that world not being Earth, and no one from other worlds, any other worlds, Earth included, are present on that one.

It's the story of a (male) healer who was made into a sex slave for the heroes (male and female), who decides to flip things around and go on a rape-quest against all who wronged him. Highly offensive, high impact sexual violence, and beloved by many female anime fans with questionable sexual fetishes. Tourists, however, treat it like vampires treat garlic-coated crosses, because... rape-quest.