I ought to promote you here and now, but that wouldn't be fair to the others
Here's my submission:
It was a cold wintry night in the city of Snowdin. Well, colder than usual. All of the gentle-hearted monsters hunkered in their houses to avoid the cold, enjoying steaming bowls of dog food with sliced hot dogs in the mix. Rabbits, skeletons, wolves, and lizards alike enjoyed the opportunity to lounge about without the incessant pangs of guilt that usually came from extended periods of laziness.
Somewhere outside of the city...
Although some might mistake him for a monster, he came from a world quite unlike the realm of monsters. Dungeon Man, previously known as a dungeon developer, a man so dedicated to crafting dungeons that he chose to become what he loved best, shivered with his stony arms in the cold. Last he remembered, he was standing in front of two palm trees, not sure what to do next. Next thing, he looked up, down, and there had been snow. Snow so cold that it bit into his mazey center, sending its walls into an uncontrollable shiver. He ventured toward the city in the hopes that a kindhearted resident might set up a fire for him; no house could contain his immense size.
Somewhere above the city...
Rainbow Dash, after performing a quadruple rainboom whilst hyped up on coffee, broke all laws of physics at each instantaneous moment in time as she traveled across an infinite amount of dimensions to land, almost impossibly, in the realm of monsters. Even more improbably, right above the city of Snowdin. She took a moment to get her bearings, found that she'd gotten very lost, and started crying.
Somewhere below the city...
One punch man, after punching through the ground looking for that mole creature that had so rudely assaulted him, continued to punch into the ground until he ended up reaching the surface of what was now Snowdin. He scratched his head in confusion, because it usually took him a bit less time to reach the center of the Earth and come out again on the other side. Many of the S class superheroes didn't seem to realize that flying from destination to destination could be a lot slower than burrowing through the earth because of the arc you have to follow from point A to point B. But that was none of his business, as a B class hero. He scratched his head again because this didn't look at all like the Earth, and in the distance he saw a mountain walking, and above him a crying pegasus.
In the time it takes for Temmie to sneeze (as well as raise college funds), one punch man had gathered the three of them into the center of the city, created a bonfire, and started a fire by waving individual logs through the wind so quickly that the friction created by the air made them spontaneously combust. Keep in mind that Snowdin is cold, and on this night, it had been unusually cold.
Warming up to the fire, and to their new company, the characters talked.
"I like this place," said the bald man.
"Me too," said Rainbow Dash. "The ponies here are nice. But I guess I shouldn't be calling you ponies."
"I'm a dungeon," said Dungeon Man.
"I'm not," said the bald man.
"Oh," said Rainbow Dash.