The story could be the Two kinds main character(also included Eric and his slave) been put into a room and had been tell that they will react to a musical from another world. And they will be tell that the whole musical was a real story not some made out fairy tale STORY things.
Musical will be start with the pacifist first and the genocide came after. Gaster musical also will be included. Just put their reaction about it.
ps: I had this idea because the undertale world also happen of the war between two species, so it may very fit to made them react to it.