"And that's the last one" Twilight stated as she slid the last book in place in her library. She and Spike were spending the weekend organizing books in her castle, one of Twilight's favorite activities.
"Phew, now that we're done, can we please go eat something?" Spike complained. "We skipped breakfast, and I'm craving some diamonds."
"No, silly, we have so much work to do! We have to clean the rooms, check the map, restock the kitchen…"
As Twilight rambled on, Spike felt a familiar rumbling in him. It started at his feet and traveled up to his throat. He knew that feeling.
"Uh Twilight…?" Spike mumbled.
"Dust the curtains, mop the floor…"
"Twilight?"
"Repair the windows, do the laundry…."
"TWILIGHT!" Spike yelled.
"Spike! You don't have to yell!" Twilight admonished, just as Spike finally burped the letter that was causing the rumbling.
"Hmm, what's this? Well what else, a letter from Celestia, of course." Twilight mumbled to herself. She picked it up and started studying it like a textbook. Whatever was written in it must've been serious, because reading it made Twilight visibly pale.
"Well, what does it say?" Spike asked.
"It's asking, no summoning me and my friends to The Castle of the Two Sisters" Twilight read quietly. "Celestia only summons ponies there for serious reasons! What did I do? Was it something I said? Something I did? Am I in trouble? Did I fail my princess duties? What did I do?!" Twilight shrieked. She started to hyperventilate.
"Relax Twilight, it's probably nothing. Maybe Celestia wants to congratulate you on your mentoring of Starlight." Spike reassured.
"You're right Spike, it's probably nothing" Twilight sighed. "Well, let's go find everybody and head to the castle.
Twilight gulped as she stood in front of the Castle of the Two Sisters. Even after it was renovated, it was still an imposing sight. It's three towers rose high, and Twilight had to step back to see the full view.
"Twilight, looking at the castle isn't going to help. Just go in." Spike urged. Finally Twilight sighed, and opened the door.
The main foyer of the castle was spacious, and was empty save for some banners, a carpet, and two thrones, on which Celestia and Luna were sitting on. Twilight saw her friends, and trotted towards them.
"Come in Twilight." Celestia called. Her serene voice made Twilight feel calmer, and she trotted forward.
"Why have you called me?" Twilight asked. Her anxiousness returned. "Did I do something wrong? Have I failed my duties as princess?"
Celestia giggled "You have done nothing wrong. In fact, you have succeeded, in your duties as princess." Celestia turned serious. "You and your friends are here for a more serious reason. You see, Luna and I owe you an explanation".
"An explanation?" The Mane 6 chorused. "For what?"
" How many princesses exist in Equestria?" Celestia asked.
"Five" Rainbow Dash answered quickly. Rainbow Dash liked speed.
"Well, that's where we owe you an explanation" Luna said. She flicked a lever on the wall. Immediately, four more thrones popped out of the wall, two on either side of both thrones. Three of the thrones had a cutie mark on them. A purple sparkle was one of them, and a crystal-blue heart on the other. These were obviously Twilight's and Cadence's thrones. The other throne was blank, obviously representing Flurry Heart's blank flank. The fourth throne had an unfamiliar cutie mark, a clock with a golden line spiraling around it.
"Princess, what is that?" Applejack asked.
"That, my little ponies, is the throne of the 6th princess, Temperus, the controller of time".