Luna wasn't quite used to transporting so many living things at once, so the spell had left her quite drained. She collapsed on the ground, her body aching and her lungs felt like someone shoved a lit torch down her throat. She raised her head long enough to see the others had fallen unconscious. The slow rise and fall of their flanks reassured Luna that breaking them from The Overlord's power had not killed them.
She used her last remaining strength to look in the direction of Ponyville. Just above the treeline Luna could see the top of a black force field that had been erected over the small town.
The scene of Celestia being so ruthlessly murdered played over and over in Luna's mind. The loss of her fallen sister was almost too much for her to bear. Tears came to her eyes, her grief threatening to burst out of her. The same being that taught them everything they knew and gave hope to those without any, just killed one of his own students without any hesitation or remorse.
And They knew this would happen, Celestia knew The Overlord would kill her. In fact Celestia counted on it. She knew The Overlord's goal was world domination, and in order to do that he would have to eliminate the alicorn leaders of this world, starting with her. So Celestia taught Luna the spell that would harvest the Elements of Harmony from the Tree and also how to wield them.
Luna looked to the sky, which had taken on a reddish hue. Another alarming thing was the Sun was beginning to implode. It was soon a fiery blotch in the sky, rather than the spherical perfection Luna had always known. A few years back she would've rejoiced at the sight, now it filled her with despair. Luna hoped that the messengers would see their cue and warn the other cities of the evil threat.
Luna remembered the last time she spoke directly to her sister. It was just before their departure to Ponyville.

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"But sister this is good thing! He's come back!" Luna protested.

Celestia raised a hoof to calm her little sister, she had to try to make her understand.

"Luna, you know it's not actually Him."

"But His magic, it's the same-"

"Luna!"

She fell silent and dropped her gaze after that. Luna knew that when Celestia only raised her voice when the situation was truly dire.

"You felt a small blip of his magic among a pool of darkness. Whatever lies in Ponyville is not our former mentor. We must be prepared for the worst."

"Sister I don't think I can-"

Celestia put a hoof under Luna's chin and raised her head to meet her eyes.

"Little sister, I know you can."

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The memory pained her, she tried to forget everything, to become lost in a sea of agony. But her mind refused to give up. She was still alive and she still had a purpose. She looked again to the fiery explosion in the sky. Celestia's magic was fading from this world fast. It was only a matter of weeks before the heat from the Sun disappears entirely and the world freezes to death.
It was all the more reason to get to the Elements and reactivate them as soon as possible. There was still time to use the power of the Elements to reverse the effects of the darkness.
Some of her strength had returned, and the others were still out cold, so Luna raised her head and tried shifting to a sitting position to get a good look at their surroundings, but settled for lying on her stomach.
Unfortunately in her haste Luna accidentally teleported them to the Castle of the two sisters. The ancient structure had been built by her teacher as a departing gift. Now it sat as an empty husk, a mere shadow of its former glory. The Tree of Harmony was a short walk from the Castle. There was still the threat of the creatures that wandered the Everfree, but the way The Overlord so brutally murdered one of the most powerful beings in Equestria made whatever lurked in the forest seem like simple insects.
Twilight began to stir, slowly, as if waking up from a peaceful slumber. Then she immediately stood up and cried out in pain and grief. She looked around frantically, when her gaze landed on Luna, she immediately realized where she was. She tried to take a couple of steps, but soon gave up,and collapsed to the ground.
Twilight looked up at the sky, then at the dying Sun. She knew exactly what it ment but still looked at Luna, not wanting to believe that her mentor was gone. When Luna only returned her grief-stricken stare, Twilight felt the truth drop on her like a boulder. Since she was so weak all she could manage was softly weeping, but the loss of the great ruler of Equestria hit her full force. Luna tried to move over to Twilight, but the fatigue left her helpless to comfort the young alicorn.
One by one they all woke up, realizing the sad truth without any words being spoken. They cried for their lost Princess, they cried for the hopeless odds against the limitless power of The Overlord. They could just barely move, their strength sapped by The Overlord. They had to get back quickly. Discord could hold out so long against The Overlord.
Suddenly, there was a burst of green fire, then Spike fell through, and the gout of fire vanished. Most of his scales were blackened and he was unconscious. He was close enough to Twilight so that she could muster what little strength she had left and crawl over to him. She wrapped her forelegs around his charred body and laid her head on his chest.
Luna had a faint spark of hope. She had no answer as to how he got here, but they were all together.
And that's all that mattered.

Author's Notes:

Man this was a lengthy one. It was revised several times but I finally got it done. I promise the next chapter will be a lot more interesting. In fact, I'm going to break your heart with it. (It's gonna get super gory. If your not into that sort of thing then what can I say. If you can live without knowing how this story ends which I would not recommend doing, then just stop right here. If you can't, well go get your tissue box.)