I tried looking based on tags, but nothing came up.

I want to find a yaoi manga. It was in black and white, I think the story was a funny erotic isekai where a guy found himself in a medieval fantasy world and ran into the villain when he was young. The protagonist knows how everything is supposed play out and that the villain will eventually be defeated. He serves as his minion, trying to alter the course to make sure the villain succeeds in world domination or whatever the goal was. It may have been shota.

Also looking for pic related