Jack of Blades in Equestria?!

Celestia stared up in horror at the fiend, one she has never seen stood there mocking her in every way. Luna poor little Lu Lu, was beheaded at the feet of this monster. He looked at her through his multi-colored mask, his eyes were maliciously red just glared at her.

"I will not allow you to hurt anypony, I swear on my life I will find a way to stop you!"

Her response was met with the icy stare and the she knew fully was a creature of purest evil.

"You had such a playful sister, and you dared to spoil our fun!" The red hooded figure steps drew closer to her, her eyes pooled with tears. "The beasts and monsters you fear are nothing compared with me. Look at what I can do, look at what I have done. Do you really think I can be stopped?"

His monotone voice was the most chilling Celestia had ever heard. Sombra wasn't this cold, nightmare moon had never been so violent, and Discord never killed anypony. This monster was more terrifying than tartarus itself. It lowered its face to hers, running a metal finger up her neck to her chin. It pulled her face to meet those damned eyes.

"Now you will die." it's right hand drew back with a long black sword. When it connected the pain was unbearable, she screamed as the cold metal pulled away. Again it came down upon her, the flesh parted as it sliced deeper. Those who were alive could only watch in horror as the weapon did it's ghastly work. Again and again the black blade hit it's mark, blood spurt from Celestia's wound. The masked figure heaved the blade one last time down into the princess' neck severing her body from her head. The blood along the floor seeped into every space in the palace floor, the wicked sword fulfilled it's master's wish. His left hand held her head above his own, her once cascading hair was caked in her own life fluids. Those whom had witnessed this most foul act vomited uncontrollably. This monster was birthed by hell itself, how could it have done such a thing to somepony who never wanted to cause any living being harm? It turned to those who still lived. "Now Equestria BURNS!"

...

Celestia screamed in pure terror, in fact so loud the guards from all around the palace ran to her doors. 'What a horrible dream, that beast...'. Her mind went blank as she remembered it. It's piercing eyes red as blood. The red robe it wore with some kind of armor gave it a nightmarish visage, but the mask it bore on its face brought horrors to life. That thing killed her in her dream! That part was sickening even though it was only a dream. Twilight Sparkle could not know about how deeply this dream affect Celestia, Twilight may not have been in the dream but Celestia was glad she wasn't. After explaining the fact that it was only a nightmare to nearly every guard, Celestia finally told her sister what this dream had been about. Luna looked horrified and appalled by each gruesome detail, her part in the dream was short but made her cringe. The worst part was Celestia's demise, as it was described Luna gagged.

"Do you have any idea why this dream manifested?" Luna stared blankly at her older sister. Celestia gaze lifted from the floor to Luna, her eyes deepend with the tears welling up. Luna moved closer to Celestia, nuzzling her older sibling. Celestia wrapped Luna in a hug. "We hast thou best intentions in mind."

"I know Lu Lu, I thank you for your support during this trying time." Celestia moved to her throne. Her weary eyes noticing each step from her dream, with no blood. Every gruesome detail shot into her mind... The brutality of it was unimaginable, the look in her eyes unthinkable... The vivid memory stained her mind horribly, with each blink her mind flashed to that numbing vision. "May the ambassadors to the griffon kingdom enter."

...

Twilight had her library reorganized three times over this morning. Every book was alphabetized then numerically and back to genres. Spike still cleaning his corner of the large room. When Spike turned around his eyes went as large as plates, the entire room was turned over and cleaned.

"You know Twilight you might have a problem." Spike made his observant joke to be met by a Twilight's stare. "Geez don't look at me like that Twi it's really freaky, besides how much work did you do in here alone?"

"Spike you know as well as I do this place must be perfect when Princess Celestia gets here... How many times has she written a letter saying that she will be here within an hour? Not many I can tell you, so long as the rest of the girls don't show up and mess up the place I think we are A-okay."

As if on Que. a knock resonated from the wooden door, the pair stood there in dead silence for this brief second to hear the voice of whom knocked. Luna? Twilight opened her door to the princess of the night standing before her.

"Twilight we ask of thou's aid we you us?" Twilight nodded. "It is Tia she has a horrific nightmare, we cannot sit idly by and watch as our sister deteriorates before the kingdom, please Twilight thou has to help!"

Twilight was shocked to hear that her teacher was deteriorating. Without a second thought Twilight left with Luna in the carriage to the Royal Palace. Leaving her new "guest" in her basement, and Spike in charge.

...

  Spike didn't know what was happening. Twilight left with Luna, and Twilight told him stay out of the basement. What was so important down there? Going against his better judgement he opened the door. Each step creaked with his every step, the candle dimmed slightly as Spike descended down.  The whole basement was a mess: a tarp from Applejack's farm, books thrown about like Twilight was doing so much research about something, and something metal in the darkest corner. Bingo. This must have been what she researching all this time, between reorganizing the library. Stepping over the mess presented carefully Spike now beheld what it was. And to his surprise it's eyes were open and staring directly at him.