Twilight looked up at the red sky. The hour was her namesake, the brief time between dusk and dawn, but the sky was red for a different reason, and until a day ago, only she and two others knew why. Those two others were with her. Celestia followed Twilight's gaze, looking up at the sky. In the distance, far beyond their sight, was a great stone; an asteroid. From the ground, it only appeared as a bright light, growing larger and larger by the minute. Luna shuffled impatiently nearby. "It's time to go sister," she said, with a soft snort, "We have to deal with this before it causes any harm."
Celestia nodded, before turning to Twilight. "Twilight, I..." Celestia found herself uncharacteristically short of words. She'd never expected a moment like this to happen. A moment where she would end up leaving Twilight, and possibly never returning. "Just know, that I am immensely proud of you. If anything happens, I trust your judgement."
Celestia pulled Twilight into a hug, both alicorns fighting back tears. However, the moment was soon interrupted by another impatient snort from Luna. It was time to go. Celestia released Twilight and stepped back to her sister, who put a hand on her shoulder with a sympathetic smile. "We shall be back soon, I promise." Luna said to Twilight, before the two sisters disappeared in a flash of magical light.
Merely minutes later a massive explosion erupted, visible even from the ground, as the sky slowly faded to darkness as the night sky was no longer interrupted. In the darkness, Twilight could see a faint light slowly falling downwards, toward the waiting earth.
--------------------------------------------------------
After the spell transported them, Celestia's eyes opened to reveal the asteroid taking up nearly her entire field of view. Luna was beside her, already firing rays of magic at the stone, spell after spell bouncing off. She couldn't breathe, the void of space giving her nothing to inhale, but the spell was keeping her from needing to. They only had a few minutes before that wore off and the duo died painfully, unless they destroyed the asteroid and teleported back first. She began firing, herself and Luna instinctively pushing off and firing, the vacuum of space little obstacle to their experienced wings.
And yet, nothing. Nearly half their time was up and yet there was nothing to show for it. Luna's voice rang through Celestia's mind, magically reaching her through even the depths of space.
"Sister," Her voice sounded resolved, but fearful. "I can think of only one way I could do any damage... and you won't like it."
Celestia was able to guess what Luna meant, and her sister was right. She didn't like it. Unfortunately it was likely she was right. "Not just you..." Celestia replied after a pause, "Luna, to me, we do this together."
Within moments, Luna returned to her side. The two sisters gave each other a knowing look. After a moment Luna spoke. "This may be the last time we see each other with our own eyes."
"Indeed." Celestia replied with a nod, tears once again in her eyes, evaporating in moments in the void. "I love you Luna."
"And I love you, sister." Luna replied, her voice starting to waver as she choked back sobs of her own.
The sisters embraced, both resolved to face their darkness within themselves and to strike a deal, to save everything they've ever cared about. As they released each other, Celestia could already see the darkness beginning to engulf Luna. Of course, Luna had done this before, Celestia had not. She closed her eyes, listening for the voice of temptation, the darkness within.
"So it's come to this at last." Celestia heard her own voice, but crueler and more arrogant. "You come crawling to me for more power. Why should I help you? Why should I let you become Daybreaker? You cast me aside centuries ago, to protect your dear sister despite her willingness to destroy herself."
"I need you. Just this once, I admit it. I am too weak to do this on my own without your power." She found herself saying aloud, through clenched teeth.
There was a cruel, haughty laugh. "Very well."
Celestia's world suddenly burst to flame, as she was engulfed in it. She could feel herself changing, her mane and tale burning to ash before being replaced by raging flames themselves. Her eyes burned, as if she had been splashed with acid, as her eyes changed, her whites turning red and her pupil becoming an almost cat-like slit. Her teeth sharpened and extended, ripping apart her gums as she felt herself laughing, the same cruel laugh she'd heard moments earlier.
Her momentary lapse into insanity was interrupted as she felt a cold hand on her shoulder and a spine-chillingly familiar voice rang in her ear. "Oh calm down, you can gloat about all your newfound power once this rock's out of the way."
Nightmare Moon was at her side, Luna nothing but her vessel once again. And now... now she was Daybreaker. "Right." She found herself saying as she clenched a fist. "And once we're back to Equestria, then we can settle things."
Celestia wanted to scream. What was she saying? Settle things? This was a terrible plan! If she and Luna fought again, after all this, would there even be an Equestria left to save?
Her thoughts were interrupted as she and Luna- no, Nightmare Moon- raised their arms, magic flowing through them. There was a mighty flash, the backlash from their joint attack sending them both careening back before their flayed wings managed to push back with enough force to steady their movement. The Asteroid was heavily damaged, the near face shattered, and it's now molten core now exposed to the void.
"Once more!" Celestia found herself shouting, already raising her arm to fire another, Nightmare Moon just behind her.
As the second blast left her, Celestia felt that something was off. Just as the beams struck the asteroid, there was a second flash... The next several seconds seemed to happen in slow motion. Something inside the asteroid burst, releasing a massive blast of energy. Celestia gained a moment of lucidity, forcing herself to Luna and wrapping her arms, Daybreaker's arms, around her sister protectively. looking into Nightmare Moon, she found Luna's eyes staring back at her, surprise on her face before the explosion caught them. Nightmare Moon's horn flared just as the two were engulfed, warping themselves, and the blast, to the atmosphere above Equestria.
--------------------------------------------
Twilight ran to the crater that had been left by the strange light falling to the ground, her horn flaring to shine in the dark night. It had to be Celestia and Luna. They'd promised they'd make it! And they were immortal... weren't they? Her thoughts were clouded with panicked "what-if's" as she arrived, horrified to find the crater's bottom filled with debris. That is, until she heard movement nearby. "Celestia? Luna?" Twilight called into the dark night, her horn's light following her eyes as she looked for the source of the noise.
"We're here." Celestia and Luna's voices, simultaneously, responded. "Though... We don't think you'll be quite as happy to see us as you think."
Twilight laughed, starting to head toward the voice, hidden by brush. "What are you talking about? I'm just glad you're-" Alive. The word caught in Twilight's throat as she moved the brush aside, revealing not two figures, but one. An alicorn, tall and white like Celestia, but whose mane and tail burned with a strange black fire. She faced away from Twilight, her wings mismatched, one pure and white, the other black as the sky currently was. Her cutie mark though... or cutie marks as would be more accurate, were both Celestia's and Luna's. Even Luna's dark black "splotch" covered the right side behind the moon, and an unfamiliar orange one covered the left, with a golden sun on it. The figure turned to face Twilight, arms covering her chest and groin, as any clothing she was wearing was lost either on reentry or in the blast. Her eyes were different as well, one, an unfamiliar red eye, with a gold iris, and the other a more familiar sapphire blue, both with draconic slits instead of regular pupils. Her horn was an odd case, a swirling twist of ivory and ebony, as if both princesses horns had been unraveled and wrapped around each other.
"Now, don't panic..." Celestia's voice came out, before she was interrupted by Twilight screaming. The figure jumped up, covering Twilight's mouth with one hand, and pulling her close. Twilight blushed as the new alicorn's absurdly large chest pressed just under her chin. "Shhh... It's us, I swear to you. We're... in here. With..."
There was a pause, before Luna's voice emerged next. "Our darker selves, Nightmare Moon and Daybreaker, are in here as well. We will explain everything once we're back at your castle. Understood?" Twilight nodded, stunned. This was going to take some getting used to...