Princess Luna had heard of Twilight's experiences in meeting other versions of her in various dimensions, but one of those experiences has utterly broken her older sister to the point that she shuts herself out from the world. Determined to help her sister, Luna goes to Celestia to learn what has happened. Princess Celestia, in turn, tells Luna of a world where another version of her brought nothing but death and suffering to not just their own people, but to so many others in an attempt to convert an entire world.

*A sequel to a chapter in Joe Toon's story Words of Advice from the many worlds of Twilight which I recommend reading. This is canon with the NegotationsVerse btw.*