It was all fun and games until it became a reality....
Deltarune takes place in an alternate timeline, so it doesn’t take place before or after undertale. The monsters were never driven underground. Asriel lived and grew up and went to college. Undyne has both eyes and has never met Alphys, who is more into western comics and cartoons instead of anime and manga (tho she started reading Mew Mew Kissy Cutie recently). Bratty and Catty aren’t BFFs, they hate each other. Mettaton is still a ghost. Toriel and Asgore got divorced for probably normal reasons. Gerson Boom and the Shyren are dead. Sans fucked your mom.