One late afternoon in Canterlot ..
Luna had just returned the books that she had checked out from the library, and was about to look at one she was saving for last. She and Celestia were going to look at it together. They had started reading to cope with their sorrow over losing Starswirl and the other pillars. They had all died of different causes and were buried in an undisclosed location. Stygian was still alive, but his whereabouts were unknown and they had no way to summon him. After stopping at the shelf, she gently pulled down an extremely worn and leather covered book. It was covered in a layer of dust on three sides, and looked very fragile. This book was so old, that it was recommended to put it in a box while carrying it. After returning to the palace, Luna arrived in Celestia's room and opened the box. Celestia eyed the book and saw that there was no title on the front cover. Luna took a deep breath and blew off the dust. She inadvertently blew so hard that the book was whipped open and dust was thrown in Celestia's face.
"ACHOO!", Celestia sneezed.
She ended up sneezing so hard that Luna's mane was ruffled by the wind. Luna shut her eyes and opened them when the gust was gone.
"Sorry, Sis. I guess I blew too hard.", Luna said quietly as she fixed her hair.
Celestia forgave her sister and then saw something peculiar about the book. There was a small lump behind the page on the right. Lifting it over, the sisters saw that a compartment had been cut into the book. In this small opening, was a crown and a parchment envelope. They quickly realized that both the crown and the writing on the front of the envelope could only belong to one pony they knew all too well: their mother, Queen Galaxia.
The writing said: "To my two babies, Celestia and Luna".
"I wonder what Mom would have hidden in here.", Luna thought out loud.
"There's only one way to find out.", Celestia said as she opened the envelope.
Inside was a piece of parchment that had a note written on it.
"My dears,
If you're reading this, then pay attention. Long ago, when I was a filly, the world was at war. Equestria had not yet been founded. I happened to overhear the six pillars discussing the construction of an underground room, in which to store the most precious of valuables ever known to exist. After the founding of Equestria, the pillars relayed this information to the leaders of the three races. Over time, artifacts that were either crafted, traded, or stolen as war trophies were placed in this cavernous chamber. All I know about the location of this place is something I heard from Starswirl; "The secret lies on top of the world." Since I had my hooves full after giving birth to you, I do not know how large the chamber is, where it is, or how much is in there. I feel that this could be the largest treasure in Equestrian history. Who knows how much it could be worth? If you happen to find it, construct a museum and place everything inside for all to see and learn about our early days.
Love, Mom."
Luna and Celestia were speechless.
"Why did Mom never tell us about this?", Celestia wondered.
Luna guessed that they may have been so young that the story would have scared them. Celestia felt that Luna was right. They sent a scroll to Twilight, telling her that she deserved a break from being sent on missions, and that they would be summoning the Young Six in the morning. They also asked her to send a scroll to Flurry Heart, telling her about the upcoming mission and to gear up. They explained what was going on, and Twilight agreed with them that she could use a rest.
"I think we should send Neighsay as well, he could use some time to get to know the Young Six a little better, and he is about to learn more about our history than he could ever imagine. He shall be one of the first.", Luna suggested.
"Good idea, sister. We shall inform him in the morning.", Celestia replied.
They split up and went to sleep for the night.