Once upon a time, in the magical land of Equestria...
If the adventures of Twilight Sparkle and her friends have proven anything, it's that the spirits of ponies are, if nothing else, resilient. No more than a few hours after Tirek's defeat at the hooves of the six friends did the citizens of Equestria begin rebuilding their respective communities. Looking at them, one would think it perfectly natural for ponies to spend as much time repairing their buildings as they did actually living or working in them. In that light, the residents of Ponyville were especially lucky; they had the pleasure of a new crystalline palace at which to marvel as they went about their daily business of plowing debris, patching roofs, and planting trees.
Speaking of business, there was still the issue of what would become of the library, which was sadly damaged during the battle. Still, the value of a walk and a little solitary reflection was obviously not lost on Twilight as she trotted down Mane Street, towards the path that would eventually carry her into the Everfree Forest, and beyond that, The Castle of the Two Sisters. The aloof, yet majestic ruin had been an endless source of memories and knowledge for the Princess of Friendship, and on this morning, there was one particular memory she wanted to revisit, for this one had taught her some of the hardest lessons she had ever learned, both about magic and friendship.
As Twilight walked, the still-fresh memories of Tirek, the palace, and her new title all bobbed in and out of her head, mixing with details of the memory she was trying to recall. The swirl of faces, the vivid recollections of every emotion she felt, and a thousand other little details all seemed very relevant now. Like the whole affair with Tirek, the events of this memory had imparted no shortage of wisdom to her about being not only a great princess, but a good friend and leader. Both had come at a time when she had felt unsure about being any one of those things, and despite the doubt that once clung to her, she finally understood the hardship and sacrifice all three roles required.
The castle loomed into sight, along with its single rope bridge entrance, which sat precariously as ever over the chasm below. Feeling very much alone and not in the least bit troubled, Twilight made her way across the bridge, past the great front doors, and at last into the library. She made herself comfortable in the hidden reading room, where she and her friends first discovered the princesses' diary, and began to stare out the stained-glass windows. As she did, in this hallowed and historied place, she came to realize all these past events had taught her something even more important: that friendship made those hardships and sacrifices well worth it.
The windows began to glimmer as the crystalline contours of their designs danced around the new sun, as if they were too excited with anticipation to touch the light. When they finally did, the simple pictures of stars, clouds, and mountains lit up in color, and in a brilliance too garish, too perfect to be seen anywhere else besides some fantastic dream. The spectacular display invited Twilight's mind to gently wander, and drift back to when she was still a neophyte princess, on a mission that brought her and her friends into the depths of the castle, beneath the foundations, beneath the Tree of Harmony, and into a strange and ancient prison constructed by two of her fellow princesses.