In the beginning of the manga, it is apparent that our main character is overpowered. I do not remember if he was reincarnated but I would say that is a strong possibility. After getting a quest to subjugate a dungeon, he goes in there and some of the other parties that went in just hours before are already dead or dying. He makes it to the end of the dungeon and starts to fight the demon that is its boss; however he cannot defeat it because the demon is actually the dungeon itself. It is later revealed that the demon that was the dungeon was not actually its true body. But the demon that did have this power ends up coming to our main character because she needs his help.
The offer she gives him is that she will give him a dragon that she has tamed, but he does not want the dragon; rather, he gives the dragon's ownership to a female companion who is kind of weak. Later in the manga it turns out that the (blonde if I remember) hero is not doing his job; rather he requests women all the time and never actually does work. Because one of the females that our main character was with was summoned by this scumbag hero to be his, our main character goes and beats the crap out of him. While doing this our main character is disguised as the Demon Lord and while leaving with his female companion he put a spell on the hero that makes him feel pain whenever he touches a women.