"It's so nice to see you again!" said the thing that was not Celestia. "Oh yes, I know who you are, Twilight Sparkle. I had a taste of your magic the first time we met. Mortals forget things so easily, I find. Allow me to refresh your memory: you tried to kill me. But you failed, Twilight Sparkle. You only mostly killed me. And I guess that makes you one very unlucky little pony."

Luna discovers something hidden from Celestia for twenty years, now, with emotions running high, she enlists the help of the Mane 6... but what seemed to be a single mystery, soon reveals a darker and much more complicated plot that will soon decide the fate of Equestria.

Check out the Empty Room TV Tropes Page (Warning: Spoilers!)

Revised chapters edits and proofing (with many thanks) by: lammy, Trevor, Nick Nack, Fifth Alicorn