After a neutral run by Frisk, the core breaks down, causing monster civilization to crumble. Alphys theorised that Sans's power could fix it, so Undyne (the new queen) smashed his skull and stole his bad time eye. However, it didn't work, so now 7 years later you have Horror Sans. Then a new human, Aliza, falls into the underground, but Chara is still there, so Chara immediately takes full control over Aliza, and performs hundreds of genocides, but the resets only bring it back to when Aliza falls, not when Frisk did. Suddenly, Gaster glitches back into existence, only to be immediately attacked by Horror. Horror believed that if he could kill Gaster and take his power, he could fix the CORE, after all, Gaster was the one who built it. Gaster soon realised that the only way to save himself was to give Horror a different enemy, so he showed him every genocide route the humans have ever done. Horror then realised that he had a bigger enemy than Gaster, and he would stop at nothing to bring them down.
