Board Thread:General Discussion/@comment-27907368-20160323193924/@comment-34518800-20160704181115

Headcanon (Serious):

Frisk is actually dead, and the Underground is purgatory. In the war, the Humans killed all the Monsters. When they break the "Barrier", they ascend to Heaven. Except for Flowey. He's stuck there, slowly descending to Hell.