Take a story about defeating the Demonlord and flip it. You did the deed(killed the DL) and now you are stuck with the sword that killed it and it will not let you go. Now you are branded as a thief for not returning it because .. you can;t. so You go to the only place you feel you can be safe. Back to the dungeon you were before. But things have changed.

Yes, this is a switch and yes it is ecchi as the 'hero' and his friend want to turn the former dungeon of horrors into... and erotic dungeon, feeding off the negative emotions of shame and discomfort.

This deferentially seems to be written from a male perspective and you should take it as so. You will 'see' all the usual parts and they come up with some different ways of 'extracting' the magical essence's needed to run the dungeon and turn it into a paradise for them.

It has the usual tropes, maid, former foe girl turned to your side, sex with said people. So you get what you paid for.