@horse emoji
Here’s a condensed story to elaborate. Turns out I did get inspired two years later.

After many years of being Princess Twilight’s right-hoof mare in charge of daily operations, Starlight received some visceral reminder that Twilight is immortal and she is decidedly not. She soon decided to detour from her inspection circuit to the statues of the old princesses in the forest. After gathering the scattered trinkets surrounding the vine-encumbered statues, she remembered a proverb and waited for night.

Entering Twilight’s bedroom as she slept was easy enough. The guards let the top logistics mistress stroll right in. She started to unpack her saddlebags before returning the Elements of Kindness and Generosity. If the guard was asked to move a statue of a sleeping Twilight, Princess Starlight would soon be dismembered in her sleep and Equestria would fall to anarchy in a world without boss mares. Patient and silent she waited.

Eventually, Twilight rose to head to the toilet room. Starlight readied the Elements from her saddlebag and moved from under the drafty curtain to behind the door. When Twilight returned, Starlight released the stored magic to earn her promotion. Once the aura cooled to stone gray, Starlight used her new power to teleport to the statues of the elder princesses. When the guard came to investigate their prime suspect, she had all the self-defense set in stone. Beneath Celestia’s petrified hooves (easier access than risking impalement on Luna’s eternally-pointed horn), Starlight buried the Elements. Ever since readying their magic to depose Twilight, she had premonitions that destroying them to prevent anypony else from earning the coveted promotion to princess would be her downfall. She would not be found lacking when Equestria faced its next threat that even an alicorn dose of normal magic could not defeat.

After 70 years on the throne, 60 past her expected death day, a slow-motion disaster spread through Equestria. Ponies quarreled incessantly, yet Discord could not break his prison. Deliveries ran late as the distance freight pegasi stopped to take rest. Starlight became increasingly desperate and despondent as her magic waned to that of a mere unicorn prodigy.

One day, as she took a trot in the woods to clear her mind, she found herself drawn to canter deeper in—deep to where she surrounded herself with three alicorn statues, wondering if it was her turn to join them to form a complete table for Hearts or Euchre.

“Help me,” she whispered to Celestia’s inert feet before pawing at the ground.

Generosity, Loyalty, Kindness, Magic, Fun, and Honesty. All six elements gleamed in the pristine arboreal shadow. Starlight willed her mane to wrap around Celestia’s neck and stone approximation of a mane, then lifted her front right hoof to caress Celestia’s eternally-raised left hoof. Wings flared forward to embrace the statue as she levitated the Elements to will life energy to return to the former monarch.

Muted cracking and chiseling, no louder than rustling leaves, filled the cavity between princesses. Celestia looked at the purple alicorn who betrayed her many moons ago.

“Why?” Celestia whimpered.

“I need help,” answered Starlight.

A final moment of embrace broke as Celestia levitated and activated her royal voice.

“YOU STONE ME TO RUN EQUESTRIA MORE EFFECTIVELY AND NOW COME CRYING TO ME TO HOLD COURT? RETURN ME TO LITHOMORPHIC SLUMBER. PONIES ARE SINCERE WHEN I’M IMMOBILE ROCK.”

Shielding herself in a magical bubble, Starlight cast an illumination spell on Twilight’s stone form.

“SO YOU WENT PLAYING WITH PARADOXES. EQUESTRIA IS PROBABLY ALREADY TOO DEAD FOR ME. GO AWAY.” Celestia managed to become even less impressed at her former prize protégé as her mane shifted from pastel to fire.

Starlight flapped to place her flank alongside Celestia’s face. “Will you look at my cutie mark?” She screamed, ignoring the pain that was sure to follow as her leg got singed.

Confusion cut the anger to dim the heat radiating from the sun. The lavender alicorn before her had only the single set of overlapping four-pointed stars, two fewer than the pony who stoned her.

“What strange princess are you?” Asked the reawakened monarch.

“Look beside me.”

Celestia glanced at the stone alicorn beside her sister. Even the mossy rock showed the now-hated six-pointed star on her flank. Instinctively, she spun in place to launch a double-barreled kick at the head of her former star pupil.

The marble horsehead and mane implanted themselves into the dirt with the sickening thud of a grown mare going splat.

“Why did you wake me?”

“Friendship is on the decline and magic is weak. I need your help to restore Equestria.”

“You need our help. Restore my sister.”

Starlight levitated Kindness, Honesty, and Generosity toward her predecessor. “Together, as a sign of goodwill?”

Celestia grabbed the Elements in her golden aura and the two mares combined magic as they focused on Luna. Starlight heard the cracking once more, as the night princess began to emerge from her concrete prison.

What was once left was now up when Starlight realized what happened. She lay on her side as Celestia wrapped a wing around a concussed Luna. She must’ve had a stored charge ready to launch while being petrified that now discharged at her saviors. Starlight stumbled to stand up, only to find her left front hoof clipped straight through the ground. She tried again with the same results. The spell must’ve smashed and reanimated Twilight, judging by the disembodied purple hoof and pastern on the ground.

Then why was the statue of Beheaded Twilight still intact? Was there a changeling attack?

“Your leg!” Celestia neighed in alarm at the sight of her rescuer’s missing appendage. Risking stains to her pristine coat, she inspected the stump. Full-power Alicorn battle magic had enough spare energy to cauterize the wound.

Celestia then directed Starlight to try to talk to Luna while she flew off to summon three squads of pegasus medics.

Starlight found herself unsuccessful at conversation with the Princess of Night. She occasionally got grunts and nickers, but it was mostly unfocused staring and silence. She tried crawling over, but an alicorn’s frame was too much for only three legs. Luna eventually grazed at the thin grass and made her way toward Starlight.

After an hour, the medics finally arrived, along with Celestia. They loaded the two incapacitated princesses on oversize stretchers and took to the air. Celestia flew between them, mostly making conversation with Luna’s attendants. The snips of conversation Starlight overheard were that benig sealed in stone with all the magic summoned must’ve fried Luna’s brain. A lesser pony would’ve died, but enough of Luna’s stem remained intact for her body to shamble on. Whether she was gone gone or merely forever in the dream world is unknown. Perhaps she may even return. After all, she hadn’t already died—it was not her entire life energy that blasted Starlight’s leg off.

Celestia briefly returned attention to Starlight upon landing in Canterlot. “Yes, I will do what it takes to restore magic to its proper levels.” She glanced at her sister. “If there’s anything that could cure her, it would be the return of ambient friendship. Even if not,” she paused with a sob, “her body would no longer need to pretend she’s home.”

I guess the Elements are a little too loyal on Twilight’s side here.

Starlight: “What happened!?”
Twilight: “Celestia and Luna have just resigned. I am the new princess of Equestria.

They always did like the royal garden. Put them in it!”

@Harshie83
TS: “I have good news.”
QC: “What good news could you possibly have for us?”
TS: “I no longer intend to imprison you.”
Tirek: “Wait, that is good news, right?”
CG: “No, it means we’re fucked!”

And then Twilight kills them.

So, this is pretty much canon to IDW comics, considering what was revealed in the latest issue.

@Ereiam
If not for the following two reasons, I’d use this image as a motivator to write a story. However,

@PUBLIQclopAccountant
And knowing she better keep her mouth shut about it or she’s next too.

@I, Da Witchfinder
Starlight’s grimace at learning the truth of Twilight’s promotion.

I’m not sure what stings more: Celestia’s look of betrayal or Luna’s last defiance.

If Gainax made Season 9 of MLP:FiM.

Approved!

@DragonBliss99  
I don’t know why people think shock gore of Pinkie murdering ponies and turning them into cupcakes was a thing in the early days, either, but here we are. People like to take the pure things and twist them with dark themes.
 
People are lame.

I never understood why some people think Future Twilight is evil.

Twilight: betrays Celestia  
Me: Protec waifu!

All the Elements are just lying about in case Starlight wants to give herself a promotion later.

@Background Pony #E107  
why

All hail empress Twilight!

Wait don’t the elements of harmony have like a build in safety mechanism so that things like this cant happen ?
 
 

TRAITOR !  
How could you do this?

Elements of Harmony? Where we’re going we won’t need Elements of Harmony

Assuming they can see and hear what’s going on, Chrysalis, Cozy Glow and Tirek are having a good laugh.

“The princesses are dead, long live the princess!”