Luna looked over the many stained glass windows in the great hall. Many were there, Twilight and her friends defeating Nightmare Moon, them defeating Discord and them becoming the bearers of the elements of harmony as well as many more. Then, she came to a window. Nothing special about it, an empty, glass window, waiting to be filled with some sort of event by the bearers.
The door at the far end opened, and Celestia came trotting in, towards her sister, where she then stood and faced the same window.
It wasn't filled, but it was different from the rest. On the other stain glass windows, there were plain, black and purple borders surrounding it. However, this one's border, whilst still the same colours, had patterns engraved into it, and letters and words from the the ancient Equestrian language, so few could understand what it said.
Celestia said three words, three words that could change the course of history itself," It is time."
Lunas' eyes widened and she quickly turned to face her sister. "But, Tia, you can't be serious?"
"It is time," she repeated what she had just said, not acknowledging her slightly frantic sister in front of her, obscuring her view of the window.
"No, it is not. You said so yourself, we have to wait."
"It. Is. Time." Celestia turned away, and made her way towards the door.
"Tia, wait!" Luna ran and placed a hoof of Celestia's shoulder. "Tia, you are not thinking straight. Look, I know some things may have changed your mind lately, but we have to wait. There is no other option."
"We have to. It is the only way."
"Tia! Are you out of your mind? They don't want it and we don't need it," Luna tried to make reason with her sister, as she saw her pick up five rolled up parchments in her aura. "Five?! You mean you're not telling Twilight? You can't be serious. It won't work anyway, the elements of harmony are now in the tree of harmony and won't work until they are ready, which they are most definitely not."
"Lulu, it will work. All they need is a little push, one of which I am going to assist with," she replied. Her horn lit up brightly, before turning slowly into each colour of the rainbow. It lit up the whole room, and Luna covered her eyes as a final spark blasted out of Celestia's horn. She then picked up the five parchment letters, and used her magic to send them off. "It is done," she said simply before exiting the room.
Luna watched in dismay as her sister ignored her advice," I need to get Twilight."
"Ya'know, ah still don't see why tha Princess wanted us to come to the Tree of Harmony," Applejack questioned.
"Well, I must say, I don't usually question the Princess, however it is a little odd she wanted us to come here without Twilight," Rarity added.
"I know why!" Pinkie Pie smiled gleefully as the rest of the main six turned to face her, with curiosity. "It's just a guess- I love guessing games- but maybe it's a surprise party! And maybe Twilight does know about it, she just wants us to think she doesn't so that it will be a surprise!" She grinned. "Wait, if it's a surprise party, we should be more careful."
On cue, Pinkie dove behind a bush, so only her eyes showed. She looked around before tiptoeing towards the rest of her friends. "C'mon girls, that's not hiding," She grabbed bushes and threw them at her friends, to which they all dodged.
"Gaaaaaaaah. My mane!" Rarity exclaimed as Rainbow Dash rolled her eyes.
"Rares, your mane is fine," Rainbow tried to calm Rarity down.
"Um, girls," Fluttershy said. They continued bickering and talking. She took in a deep breathe. "Girls!" She shouted, which to anypony else sounded like normal talking, but it was enough for them to hear her.
"Yes, Shy?" Rainbow asked.
"Oh, um, we're almost there, look," she pointed to the long, old bridge that led towards the castle of the two sisters.
"Aw, yeah. Time for an adventure, maybe," Rainbow pumped a hoof in the air. "C'mon, girls, let's get to it."
They all started walking over the bridge, to get to the castle, near where the cave containing the tree of harmony was.
"Now, if ah remember, the tree should be over here somewhere- aha! Here it is," Applejack pushed back branches and plants, and held them back for all her friends to pass, before joining them, in front of the tree of harmony.
It was exactly how they left it. Twilight's element and cutie mark in the centre of the tree, with branches weaving of it. Each branch had its respective element on it, and Luna and Celestia's cutie marks were on the trunk. The long stem that led to the flower with the box was also there, the keys still inside of the locks. The tree was practically glowing, it was so shiny.
"Hmmm, well there's no surprise party," Pinkie thought out loud whilst tapping a hoof on her chin.
"There's nothing here," Rainbow stated as she flew behind the tree, to check if there was anything there.
"Maybe we should go?" Fluttershy asked, slightly scared of the looming darkness that was around them, where the tree was not.
"Darling, I doubt Princess Celestia would send us personal letters asking if we could-" Raritys' eyes widened as a bright, coloured, glowing beam shot out of the top of the tree.
"Uh oh," Pinkie said, "I knew we should have hid!"
Their eyes widened as the beam of light shot towards each of them, lighting their bodies and manes up, and surrounding them in a multi coloured aura. A flash of light appeared, and then they were gone in an instant.
"Wha, where are we?" Rainbow asked as she looked around at her surroundings and friends.
They were in the centre of a large bright, light blue abyss. The light was coming from everywhere, it was a nice sight. They were almost floating as there was no visible floor, yet they were standing like on normal ground.
A bright light shone from above, and gently waltzed down in front of them, before turning into the figure of a pony. Celestia, to be specific.
"Princess Celestia, if I may ask, where are we?" Rarity stepped forward.
Princess Celestia smiled and took a deep breathe. "You've come such a long, long way," she sung in an angelic voice.
"Why is she singing?" Applejack asked bluntly.
"And I've watched you from that very first day."
"I knew it! We should've been more careful," Pinkie exclaimed and pointed a hoof at Celestia, accusingly.
"To see how you might grow. To see what you might do. To see what you've been through. And all the ways you've made me proud of you," she continued to sing in her high pitched voice, as she began to walk forward, beckoning the rest of the main six to follow her.
Floating were many memory's, as if they were watching them. Many moments, including the sleepover, the time when Rainbow saved Scootaloo, Rarity trying to attend Twilight's party whilst going to a garden party among a lot more. They smiled as the memory's came back to them, Rainbow stopping was a moment when she saw Scootaloo admit her fears to her, when they were out camping.
"It's time now for a new change to come. You've grown up and your new life has begun."
"New life? Nothing's really changed, I still work in the weather team!" Rainbow groaned, before Rarity glared at her.
Celestia subtly rolled her eyes. "To go where you will go. To see what you will see. To find what you will be."
Celestia stopped and turned around. Her horn glowed brightly and she slowly spread out her wings and started to glide upwards. "For its time for you to fulfil your destiny," she held the last note of her song. Her horn glowed even brighter and a beam of light shot towards the main six and they shut their eyes. They opened them...and nothing had happened.
"Um, was that meant to do anything?" Fluttershy asked.
"What? That should've worked! You should've turned into alicorns," she said in dismay. "Why didn't you accept your destiny?"
"Because it's not their destiny," a familiar voice answered. It was Princess Luna. "Tia, I tried to warn you, it would not work, but you didn't listen."
"B-but, why didn't it work? The vision I had and- wait, how are you even here?"
"I'd rather not go into the long agonising story of how I had to persuade Twilight to burn a book and get Discord to use chaos magic, when I specifically told him not to last week," she shook her head. "As for the vision, it was wrong. I sensed it, I tried to tell you but you didn't listen. Even if it was their destiny, it wouldn't have worked. One must accept their destiny before they can fulfil it. They did not accept it."
"Why though?" Celestia asked, her lip quivering slightly. Luna looked at the ponies.
"Celestia, I'm not gonna lie, being an alicorn would be super awesome. I mean, I could fly and do magic. Just imagine all the tricks I could perform! But, I wouldn't be able to become a Wonderbolt. Being a Wonderbolt would mean everything to me, I'm not going to accept defeat until I've tried better than my best. I can't crush the one dream that I've had my life," Rainbow said meaningfully.
"Um, well, I don't really want to be an alicorn. Too much attention, I'd much rather stay at home, in. Ponyville with my best friends and animals," she smiled, sheepishly as a small smile spread across Luna's lips
"Being an alicorn, would be super cool and amazing! Buuuut, I'm not sure how Mr and Mrs Cake would react if I suddenly became an alicorn. That, and I want to throw parties! Not attend boring social events," Pinkie smiled.
Celestia looked at Applejack and Rarity.
"Ta be honest, Princess, I don't think being an alicorn would fit to well on a farm. We do things the old fashioned way, so I'd be no use having wings and a horn. And I don't wanna do al that stuff which involves fancy froo-froo dresses, no offence Rarity, it jus ain't ma thing," Applejack explained.
"Princess, I-I don't know what to say. Being an alicorn would be the most amazing thing imaginable, but, I'm sorry, it is just so unladylike for me to be taller than my future stallion. It just wouldn't work. I doubt a dress business would work either. What would become of the Carousel Boutique? Like Rainbow Dash said, I can't stop my one dream since I was a little fily. I'm sorry," Rarity apologised.
"Celestia, I know you were worried about your vision, but please, please, talk to me first before making any slightly brash decisions?"
"Oh, Lulu, I am so sorry, I just, I just..."
"Don't worry about that sister, what I would worry about is that we need to get out of here now. I don't know how long Twilight can hold her magic," her horn glowed lightly and her eyes turned white. They were all transported away from the place of alicornhood, and were back outside the tree of harmony.
"Girls!" Twilight ran up to them and formed a group hug.
"We should get going," Celestia said as she glanced at her sister who smiled back. The mane six smiled and waved as the two princesses exited the cave before returning to their group hug, knowing that together, nothing bad could ever happen to them.
Author's Note
Huzzah, this is another story for you people 
I hope you enjoyed it, I've been wanting to write it for a while. Please point out any mistakes you notice, thanks for reading! 