The second fallen

this book entails the events after the true pacifist ending, where the hero of our story stays with Toriel, and lives to befriend the monster down below. But, a second child has emerged from the from the ashes of the soul residue, and begins to slaughter monster one by one, creating a bloodbath in the process. its up to Sans, Papyrus, Frisk, and the others to stop this menace before it reaches the surface and kills both monsters and humans alike. will they stop it in time? or will the genocide emerge once more...