It is related to Underswap, but takes place after a neutral run. There was a food shortage. Now the monsters have to kill and eat humans to survive.
Sans in Horrorswap, Acts like Horrortale Papyrus.
Papyrus in Horrorswap, Acts Like Horrortale Sans.
Asgore in Horrorswap, Acts Like Horrortale Toriel.
Alphys in Horrorswap, is the queen of the underground, and also acts Like Horrortale Undyne.
Undyne's location in Horrorswap is unknown, like Horrortale Alphys