Undertale is post-scratch Deltarune. To defeat the Roaring, the Lancer Fan Club is gonna have to do some shenanigans and make a new universe where instead of living together in segregated harmony, the Humans and Monsters went to war with each other, causing the monsters to be sealed underground.
Mount Ebott is near Hometown. The city you see in Undertale is Hometown, but in the Undertale universe, it was occupied by Humans instead of Monsters, who built it up into a city instead of a small town.