There's an enemy called Shyren that you can hum with to start a concert. If Papyrus is alive, then Sans will show up to sell tickets to the concert. If Papyrus is dead, instead a hooded figure will. >>> woah gotta do that
K so I've heard that the genocide/no mercy run (idk if they're different ok) is where characters possesses frisk so consider: characters making frisk kill sans and then temporarily letting go just in time for frisk to watch him die.