"Suppose the Final Boss left the last dungeon to settle into a peaceful life in the country."

"They called him the Goblin King, ruler of the Hellish Labyrinth and Bringer of the Apocalypse. It was said that 10000 years ago a great hero gave their life to seal him away with in his own creation, The Labyrinth of Anguish, and thus stopping him from ending the world. For millennia he stayed sealed away, trapped in what had become the most dangerous dungeon in all the realms, a final test for those wishing to truly test their strength. Many had attempted to face him only to be lost to the labyrinth or fall to one of the many monsters or traps that filled the place, their screams echoing for eternity. One day however, the great hero reincarnated decided to finally put an end to the Goblin King herself. She traversed the maze, defeating every enemy with her wits or her skill, disarming or avoiding traps all together, and after nearly a full year she finally made it to the center, his ultimate lair. Readying herself for a climatic battle...she found it empty. Dust covered most surfaces, the many legendary artifacts and treasure said to fill the place was gone, all that remained was a note. "To whom it may concern, for too long I've been a prisoner in my own creation and quite frankly I'm sick of it! Monsters are lousy companions and dungeon food sucks no matter how well you prepare it! The hero may have been clever enough to trap me, but she left me an out. So I'm taking it, it's been real. -The Goblin King." He was gone...THE FINAL BOSS LEFT THE DUNGEON? How could this be, shouldn't the world be cast into darkness? And what would the new hero tell people? Should she lie and tell them she slew him? If she told the truth people would panic, THIS IS NOT HOW AN ADVENTURE SHOULD END! But maybe it didn't have to? After all the dungeon had been easier then she'd expected, perhaps THIS was her true task? To hunt down and destroy the Goblin King, or return him to the Labyrinth. Either way a new adventure was beginning."

Simple enough, YC is a Heroine looking to prove herself by hunting down MC, the Goblin King. Of course when she discovers he's taken to the simple life out in the country her plans change. Definitely comedy heavy, but when smut starts it can go a whole bunch of different ways, both light or dark depending on how we wanna do things.