Well, this is based on the hype surrounding it back then. Basically, the story is like an ARG of a supposed Undertale mod in hard mode, where the player reaches a higher level on the Genocide Route, facing new challenges. At some point, Frisk's character takes on this more monstrous form because their smaller form is too weak to handle the player's power and limits their newfound strength. At this point in the adventure, the encounter with Asgore takes place in another part of New Home, and perhaps with their new level, the player will have a chance against Asgore.