The Return of the Fallen

In this story, years after the barrier between the Underground and surface was broken, a group of twisted and dark monsters known as the "Fallen" appears, wreaking havoc and aiming to take over the land. Frisk and their old friends recruit help to defeat the Fallen, only to discover they are being controlled by a powerful being known as the "Void." In a final battle, Frisk and their friends defeat the Void, save the Underground, and are hailed as heroes once more.