Natitale is an AU that takes place after a No-Kill route, though when the Monsters get to the Surface, it isn't exactly what they were expecting.
The Human known as Frisk jumped into the Underground to escape persecution of the Third Reich. When they arrived in the Underground, they thought they had found the only place on Earth that hadn't been corrupted. Their journey to Asgore's Castle wasn't in an attempt to escape, but instead to convince Asgore that escape from the Underground was the worst idea ever. It didn't work. When the Monsters arrived on the surface, the sky was filled with soot, and the surface practically dead. What Frisk knew, that no Monster did, is that the Third Reich, the National Socialist Regime, had won World War II. Soon after Monsters rejoined Humanity on the Surface, only the main Monsters (Asgore, Toriel, Sans, Papyrus, Alphys, and Undyne) were allowed to come out as ordered from Asgore. Soon after exiting, they were all, with the exception of Asgore, put into a "Relocation" camp.