I read this mange quite a few years ago.
The protagonist was transported to another world and tortured by his summoners. He was then forced into combat with other prisoners, however he gains black magic and when the opportune moment arrives, he escapes. He runs into a fairy; the fairy is hated because she is half human or something. The two end up having an odd relationship; I don't know if they end up loving each other because it was still ongoing when I read it. The protagonist later goes to a city and runs into a girl being attacked; the girl turns out to be testing the protagonist.
If anyone can help me find this book it would be a great help; it's one of those stories that broke from the normal isekai and I rather enjoyed it. The title had something like the (Black Devil) or (Black King) maybe (Evil King). What I do know is that it was a long title.