Where did Celestia disappear to, the night of the 1,000th summer sun celebration? Why didn't Nightmare Moon simply kill the mane six? What was The Nightmare willing to do in its hatred, and The Princess willing to do for her beloved little ponies?...
While exploring Canterlot Castle, Pinkie and Rainbow Dash uncover a room filled with stone tablets connected to dials. Above each dial is the name of an alicorn. These couldn't possibly control an alicorn's power, could they?
Despite that everypony has forgiven her, Luna still lives with the guilt of her actions and it eventually builds up too much for her to live with. Worse, guilt is only part of it.
Celestia has a dream about Nightmare Moon, while Luna can do nothing but watch her sister's nightmare. When Celestia awakens, the two sisters talk about what they've just seen.
The princesses gave their magic to Twilight to prevent Tirek from stealing it. As a result, they end up stuck in Tartarus. The fact that things manage to go downhill here says a lot about the wonders of family.
How could I have told you Celestia? How could I have explained to you that your beloved sister was now more damaged than she had been when she first returned? That her brief lapse into Nightmare Moon had left her so unimaginably broken?
With the help of her friends in Ponyville, the Tantabus was subdued...but what about Celestia? What about Luna's closest friend, a pony who had no idea what was going on in her little sister's mind?
Sisters are supposed share everything. Secrets, experiences, feelings, knowing silences... but sometimes first a silence must be broken for sisters to truly understand one another.