Free synopsis?

Nightmare

After Luna was cleansed of The Nightmare, Celestia believed that it was over, but she was wrong. The Nightmare slowly seeped into the castle and into the ground below. Eventually passing into the catacombs below, the resting place of many citizens of Equestria from ancient times.The Nightmare began to feed from the latent magic within the bones of the long deceased. Growing, regaining its strength and going deeper into the earth below. Plotting… Waiting.

Two / three years later, ponyville attacked by monsters (Skeletons and zombies) which are fought off, some ponies are injured, Princess Twilight goes missing, Rarity, Applejack, and Rainbow Dash go hero’s adventure, Fluttershy stays behind. Pinkie ...? Does Celestia send them? Or warn them not to go, but they go anyway?

They gear up with swords and such.

They enter the Everfree following the origin of the monsters.

They encounter Zecora who has some items ready for them to take with them that will aid them later, health potions and a few enchanted scrolls that Rarity can use to cast a spell that will teleport them back to town in an emergency, or if they need more supplies.

As they venture deeper into the forest they encounter a few monster, the closer to the castle they get the more frequently they find them.

Once in the castle they encounter more mobs, and slowly explore their way into the underground tunnel network. Which is full of walking skeletons and zombies, some of which are armed with swords or axes, or wearing old ragged armor.

After fight, they look around and find that a few of the dropped swords and armor are glowing, they pick them up and use up one of the scrolls to go to town, they find Celestia waiting right next to where the portal drops them.

“Hello my little ponies, stay a while and-”

“Yeah yeah, we just came back to have somepony look these over,” Rainbow points to the glowing items.

Celestia easily identifies the items, (because they once belonged to her, seeing as how the items were looted from her castle) and reveals enchantments on the weapons that help them, they replace their current weapons with these new, stronger ones.

They step back through the portal and continue on their journey.

After a few floors of searching through the catacombs, the trio is beginning to get run down and worn out.

Around the next corner, they’re surprised, and Rainbow takes a skeleton’s sword to the belly / leg / back?

Rarity and AJ clear out the hallway, and then rush to Rainbow’s side, they pick her up, laying her across AJ’s back, as Rarity throws open a scroll and they hurry through the portal, they barely get their on time, Rainbow is breathing is short and fast breaths, Remembering some confusing rhyming mojo Zecora spouted, Rarity pulls out one of the bottles of red sparkling potion and puts it to Rainbow’s lips, Rainbow takes a few panicked gulps of the potion, her breathing faster and shallower, her eyes glaze over.

“Sorry guys...” She looks in the direction of her two friends as her heaving chest calms and goes still.

“No no no...” Rarity chants as she looks at her friend.

Rainbow then lets out a loud gasp as she sucks in a lung full, a bright glowing comes from her accumulated wounds, and the large fatal wound and they begin to heal up before Rarity and AJ’s eyes.

“Ah don’t b’lievett...”

Rainbow slowly stands up and searches her body, with a happy whoop and Rarity and AJ then take a smaller drink from one of the potions, they suddenly feel invigorated as well.

Then they hop back through the portal to continue their quest.

After several hours and several close calls, Rarity and AJ eventually encountering their own bout with near death, they portal to town and back a few times, unloading old gear and equipping better gear they’ve looted, Celestia helping to identify the items, they eventually reach the bowels of a new cavernous evil looking lair, and after fighting off some very big monsters, they finally find Twilight.

Sitting on the throne, her mane glowing black with sparkling stars in it, her stripes glowing with energy. As tall as Celestia, if not more so, with demonic features, bat wngs, demonic armor.

“Well well well, {diabolical dialogue}...”

The trio learn that The Nightmare has possessed Twilight, and is feeding off of her magic to sustain her possession.

They portal back to tell Celestia of this change, and because they are down and element, Twilight’s Magic being useless while she’s like this, with a heavy heart, she tells them that the only solution is to… kill her.

Twilight is possessed and they don’t have a way to save her AND keep the ponies of equestria safe.

They step back through the portal, they try to bargain and beg, but Nightmare Twilight was having none of it.

Climactic battle ensues, each of the Trio nearly dying in the process, only saved by Zecora’s potions and only a moment of time to breathe.

They eventually overpower the possessed Alicorn with a trick move, Rainbow getting a fast strike in while Rarity and AJ had her distracted. Rainbow’s sword cuts a long deep wound across the alicorn’s body, The magic enchantment on her sword brings her down, nearly splitting her in half, but not quite.

The trio standing over the gasping and panting demon, the look of hatred and anger slowly leaving Twilight, her body slowly shrinking back to normal, the nightmare’s magic weakening.

“Th-thank you, girls...”

Twilight’s eyes slowly close, she lays in a pool of her own blood.

Her chest grows still, she’s gone.

The three girls all hug the body and cry.

They open the last portal scroll and bring the dead Twilight with them, Fluttershy, Pinkie and Celestia are morning.

The end.

“Not so fast, egghead.” Rainbow pulls out their last potion bottle, a small sip remaining.

She holds Twilight’s lips open and pours what’s left of the potion into Twilight’s mouth.

Everypony is watching in rapt silence.

The wounds glow, Twilight’s body pulls itself back together, Twilight sputters in confusion.

“TWILIGHT!”

The town gasps and cheers all at once.

Twilight nearly dies again from the hug of fuzzy doom.

Slice of life epilogue.

The real end.