Imagine you're Classic Sans. You wake up from previous genocide route with Chara. You leave the house. Everywhere is dust. In front of you - in a shadow - stands another Sans with Papyrus' scarf on his neck. Dust Sans. (i love it sm)
True ending: papyrus, undyne and frisk appears to help sans against dust Canon ending: sans sucessfully defeated dust but couldnt reset his timeline Bad ending: dust defeated sans and resetted his timeline