Lets start from the beginning

Celestia lay in her bedchambers, the comforter thrown wide as she rollod over and back, sweat running down her face and onto the mattress. She gasped and panted, occasionally letting out a cry of distress.

"NO!", her outburst threw her off the bed and onto the floor as her sister, Princess Luna, burst through the doors.

"Sister! What has happened!?", she furrowed a brow in concern for her sister, helping the white alicorn to her hooves before using her magic to remake the bed, "Sister, please. Let me know what it was, something about this troubles me very deeply".

Celestia rubbed her head with her hoof as a headache spread through her skull, "Luna surely you know what troubles me, that is why you came. You knew of my nightmare", she rested a thankful gaze upon her midnight-blue sister, "I hope you know how grateful I am of your presence here", then her look of relief faded away as she met Luna's eyes.

"You... didn't, see it? You didn't see him?"

Her sister just shook her head, "It was not your dreams that drew me to your side...", Luna's voice dripped with dread as she continued, "I was... Unable to enter your dreams, as there was somepony, or something keeping me out", there was a pause as the nightly alicorn took a deep breath, "As soon as I discovered this I rushed to your aid".

Celestia frowned, "Well thank you dear sister, but you may return to your chambers", a sigh escaped her lips, "I'm feel that this particular dream was just that, a dream. And nothing that I cannot handle".

Luna took a step back towards the door, narrowing her eyes at her sister before turning slowly and trotting back out of the door.

Celestia closed the door softly, a frown carved into her features, "What do you want Sombra?", her voice was laced with anger and she spoke with a venomous tone that was probably setting something on fire at the moment.

A dark mirror glinted in a shadowy corner as the white alicorn slowly padded over to sit in front of it.

Bright green eyes burst into flame in the reflection and an evil laughter rumbled fourth, "You know very well why I am here Celestia. We made a deal, and the kind of vow you took should never be broken. You know how that will turn out don't you".

For the first time in over a thousand years, Celestia flinched, and the shadowy eyes in the mirror noticed.
"Ah, yes. Your sister. I sent her away once, and I can do it again. But only if you fail to keep your promise..."

The sun princess suddenly cried out, her eyes widened in fear as Sombras' dark magic possessed her mind, tearing out her darkest fear and presenting itself to her. Vividly.

"No, Luna. No! Stop, please sister! NO!"

The next day (nightfall)

Luna was in the middle of raising the moon for the night when a knock at the door halted her progress. Emitting a slight growl in frustration, she called out to the visitor, "One moment please".

Then, with the moon in it's place, Luna trotted over to the large door to her bedchambers. A lone thestral mare bowed graciously before looking up, her copper eyes shining in the newly risen moon, "Princess Luna. Princess Celestia requests that you come to the throne room at once. She said it was urgent".

The dark blue alicorn nodded gratefully to the guard, "Thank you, Starshine", and trotted past her, heading down the white marble corridor with her star-filled mane flowing behind her as she trotted to the throne room.

"You seek my company, dear sister?", Luna trotted up to her sister, who was sitting in front of a stained glass window, the one representing the defeat of Nightmare Moon by the Elements of Harmony.

"Yes... It appears that I did", her voice was crisp and bitter, unlike any tone Luna had heard from her sister before. Celestia turned to face Luna, a dark grin curled on her lips.

The whites of her eyes were green and glassed over, smoke-like shadow poured from them at the corners.

Luna gasped lightly and stepped back, "Sister. You need to rest, you are not well", she slowly continued to walk backwards, putting a little more space between her and her sister.

The smile on Celestia's face didn't so much as flinch, "But, Luna. I haven't felt this good in CENTURIES!", her voice grew deeper until Luna realized with a dreadful feeling that this was not her sister. This was nightmare.

"You", a scowl formed on Luna's face as she stared at the possessed husk of her sister, "Release my sister. now!".

The shadow-Celestia didn't so much as flinch, "But, my dearest sister. Why would I do that?", the white alicorn's horn lit up with a faded yellow light, shadow dulling it's golden glow until it was almost grey, "I believe that I now know how you felt all those years ago", she paused, raising a hoof to her chin, "The urge to rule as one mind, and one body", her gaze drifted back to the stained glass window of the elements, the image of Nightmare Moon reflecting off of her glassed-over eyes, "And to remove those", once again she paused as the slowly turned back to Luna, "Who get in the way".

The evil Celestia lowered her horn and sent a blast of dark energy right at Luna. The smaller alicorn rolled out of the way before erecting a blue dome of energy around her.

"Celestia! This is not you! Nightmare is clouding your mind! You must fight it!", all that Luna saw was the shadowy form of her sister widen it's smile as it proceeded in blasting away at her shield.

"No Sister!", Celestia hissed, "You are WRONG! I want this! This is what it truly means to feel power! This is how it feels to be a GOD!" Her voiced raised itself to an almost deafening volume and she laughed with uncontrolled glee, watching as her continuous battering at the blue shield made the alicorn inside begin to tremble with exhaustion.

Something in Luna snapped.

"BE STILL!!!!", her royal voice shook the throne room, effectively shattering the closest windows. Celestia's possessed form flew backwards, slamming into a thick stone pillar where she crumpled to the ground.

Then, at that very moment, the doors to the throne room burst open, "Princess Luna! Stop in the name of Equestria!"

A squad of eight royal guards galloped in. Their horns down and blazing with magic, straight at Luna. The princess of the night looked frantically from Celestia's broken form to the quickly advancing guards, her eyes widened as she realized how the scene must have looked, "No! Stop! I- I can explain!" She yelled to the guards.

Celestia groaned from her place by the pillar, slowly raising herself from the floor with the support of the cracked marble pillar behind her. "No Luna! I will not let you turn my own guards against me!" The feathers of her wings were ruffled, small scratches dotted her usually flawless white coat. A small stream of golden icor, flowed down her cheek from a particularly large cut on her face.

She looked completely normal.

Luna was in a hysterical frenzy of panic, "No! But- You-". Celestia silenced her with a stare, something dark shifting in the black circles of her eyes, "Luna don't play those games with me! I can see through all of your tricks!". Luna could only stare as the two guards trotted over to side with Celestia, flanking her with their horns lowered in a defensive position.

Tears crept to Luna's eyes and started rolling down her cheeks, "Please sister! don't let yourself fall to the shadows, as I once did", the alicorn in question only stepped forward and lowered her horn, it's white surface glowing with a golden aura. Luna lowered her head and stumbled backwards as her sister's possessed form advanced, "Celestia Please! I beg of you! Stop this madness!" Her tears flowed freely now, running down her face and leaving dark streaks in her fur.

Luna listened to the hum and whine of her sisters horn as it charged up for the spell that was about to be cast. Time seemed to slow and Luna closed her eyes, she heard the dull pulse of sound as the spell was cast and the echos that rebounded around the room. She opened her eyes to find the world still moving in slow motion. The ball of yellow magic glided slowly towards her, like a comet flying through the night sky. She once again closed her eyes and focused on casting one last spell.

Her horn glowed with an almost invisible white light. Luna opened her eyes again to see the ball of energy almost directly upon her.

Then it collided.

All that was left was a dark spot on the ground where the princess of the night, Luna, once sat.