Years Ago, Earth was ruled by humans and monsters, over time a virus came corrupting monster turning them into savage, human hating creatures. W.D Gaster and his brother Calibri Gaster worked with humans to create a cure. The cure worked but left it's look on monsters making them seem as "demons", humans would say. Monsters and humans decided to send monsters to Mars but then a disagreement with monsters ended in war. In the end, the humans decided to send monster to Mars as there punishment.
Frisk was the first child, they died by through themselves into space without a suit, Asriel jumped out for them and accidently asborbed their soul, Frisk took control of the shared body and flew to Earth, Frisk knew they made a mistake but it was too late, Asriel flew in time to die in fornt of his parents. Toriel and Asgore ended up divoricing with Toriel going the Lunar Ruins. After on, multiple children ended launched to the Red Ground and never came back. Chara, a human with determination ended up on a spaceship worked on their father and launched through the Stars, They landed on Mars to get help but ended up finding monsters.
The Rest Is History