The Chaos Chronicles

Princess Luna stood and watched over the ponies of Equestria from her tower's balcony in Canterlot. She looked up into to her night and admired her handiwork. What she was able to do with the stars was breathtaking. Luna felt more accomplished about this night than almost any of her other nights. It was one of the best nights she's created since Celestia let her take control of the moon once again. It had been almost two years since Twilight Sparkle and her friends vanquished the being she once was. The Princess would always be grateful for what they did that night.
"My, what a lovely evening you've created Luna."
Luna turned around in shock to see Discord leaning against the balcony door's door frame.
"It's almost two in the morning! What are you doing up here Discord?" Luna asked.
"Oh the real question is what will YOU be doing here tonight." As he said this Discord started walking towards the nervous Princess. He put his hand on her forehead.
"Discord, what are you doing?!" Luna legs might as well have been cemented to the ground. Once Discord laid his hand on her she was frozen in place. She could feel something metalic start to encase her head. It was a helmet. No ordinary helmet.
"Discord stop! You can't do this!" Luna was fighting with all her strength to form words at this point.
"Yes I can actually, that's why I'm doing it." Discord said with a hint of felicity in his voice.
Luna could feel her teeth sharpening and elongating. "Guards! Guards HELP"
"Oh Im afraid they won't be able help you now, you see, they're a bit tied up at the moment." Discord couldn't help but laugh as he watched Luna look past him and into the tower to see two guards tied to each other and gagged.
Luna couldn't fight it any longer. Tears started to roll down her cheeks.
"Shhh, it will all be over soon." Discord said as he strengthened his grip.
The only things that could be heard at this point was the muffled yells of the tied up guards and the occasional sniffle from Luna.
Discord released his grip and snapped his fingers. "So, how do you feel?"
"I feel incredible."
What answered Discord was not Princess Luna.

It was Nightmare Moon.

Celestia slept peacefully in her bed. Up until she was awoken by a frantic pounding on her door.
Celestia rose to a sitting position and opened the door with her magic.
"What in Equestria could you possibly need at this hour?" Celestia said to the panic ridden guard at her door.
"Princess Celestia! Chrysallis has returned, she and her changeling army are attacking Canterlot as we speak, Discord has brought back nightma---" The guard was cut off by multiple black blobs that tackled him to the ground and carried him off out of sight.
Celestia shot out of bed and immediately ran to her window. What she saw was horrifying. Changelings were swarming her beautiful city. She stood and watched as war erupted in front of her. She saw her royal guards along with the guards of Canterlot desperately trying to protect her citizens. She looked to the streets only to see filly, not even old enough to attend her magic school, get mauled to death by a group of changelings.
It was sickening, the disregard for life shown by the changeling army made her nauseous.
"Im going to put an end to this!" Celestia declared.
"Don't kid yourself sister."
Celestia's head whipped around to see Nightmare Moon, standing in the room with her.
"Did you miss me? I sure did" Nightmare moon nonchalantly said.
"But...How?" Celestia was dumbfounded.
"Oh I got a little help from Discord." Nightmare moon said.
"What? No, that's impossible! He's changed! He's my friend!" Celestia felt like she was living a nightmare.
"Changed? Ha! You could have more luck changing a rock into a pillow. And that so called friend of yours despises every little thing about you."
Celestia stomped her hoof onto the ground. "Alright Luna, I've had enough! You can stop playing your little dream game, this isn't funny! I know this isn't real."
"Oh trust me, this is VERY real." With that Nightmare Moons horn started to glow, and from it, a beam of dark light was shot, grazing Celestia's shoulder.
"Aaaah!" Celestia exclaimed as blood started to drip from the wound. Celestia gathered herself and shot a beam of bright light back at Nightmare Moon. Nightmare Moon shot her own beam of dark light at Celestia. The two beams hit each other and stopped at in equal distance between the dueling sisters.
"You can not win Celestia, I am too powerful, even for you!" Nightmare Moon's beam started to over power Celestia's and creep closer towards her.
"I will not stand her idle as my people suffer!" Celestia mustered up as much magic as she could to put into her spell. She started to sweat from contration. Her horn started to glow vividly bright. When she was ready she broke the beam she had been holding up and jumped to the left to avoid Nightmare Moon's beam, which had barely missed her and broke a hole in the wall behind her.
When Celestia was ready, she unleashed all the magic she had stored within her horn in one blinding blast. The shot had contacted its target, who lay motionless on the ground.
It was only a very powerfull paralysis spell. Celestia could never find it within herself to kill her own sister, even with the given circumstances.
With Nightmare Moon taken care of Celestia ran out of her room and into the palace. There she found a member of the royal guard thrusting a spear through the chest of a changeling.
"You there! Guard!"
The guard quickly turned around to adress his princess. "Your highness!"
"I need you to tell every surviving member of the Royal Guard and Canterlot Guard to find as many citizens as they can and escort them to safety, we'll all rendezvous at the Crystal Empire."
"At once your highness!" The guard took off running.
Celestia ran out into the court yard. Once she was outside she flew up into the air to look over her city. By now she could see that most ponies have already fled the city and have started to flee towards the Crystal Empire. What she also saw was all the blood that covered the streets. The blood of the old, young, and innocent citizens of Canterlot.
Celestia couldn't take anymore. She started to fly in the direction of the Crystal Empire.
Before the city was out of sight she gave one last look towards Luna's tower. There she saw Nightmare Moon flying to meet up with Chrysallis, Queen of the Changelings. Chrysallis was standing next to someone. When she was able to make out who it was she furious.
It was Discord.