[Flashing Lights, SPOILERS] Deltarune Chapter 3 - Full "Sword" Route

...? The "Sword" Route is a secret side-quest in Deltarune Chapter 3, being an equivalent to the Weird Route from Chapter 2. You're required to do it in order to unlock the Shadow Mantle, an Armor item that's able to resist most of the attacks from the end-boss of the chapter.