Several key players were missing during the battle with Tirek in Las Vegas; Princess Cadence, Shining Armor, Twilight Sparkle, and Applejack. They had their own problem to deal with, in the form of a Spirit like Discord.
Kathy Thorndale is just your average sixteen year old teenager, until one day she starts hearing voices inside her head. Voices that could cause her to descend into madness without anyone catching onto what troubles her. Voices that she's sure belong to a character she created.

(This is a side story to the main Nightmare)