While some isekai protagonists get transported to new, fun worlds, the grass isn’t always greener. Some isekai worlds are dark, brutal, and unforgiving, making the main character wish they could go back home. Dark isekai is a newer addition to the wider genre of isekai, and it doesn’t have too many entrants. Most isekai series focus on the brighter side of life and give their characters fun powers in their vibrant new worlds. Dark isekai do the opposite, usually leaving the main character in a harder environment than they can initially handle.

https://screenrant.com/10-darkest-isekai-anime-series-every-fan-needs-to-watch/