This story is a sequel to Antipodes

Several decades after the heroic actions of Jigsaw, Tiptoe, and Incendia restored the princesses, the world has begun the slow process of healing. Spectrum is a headstrong young stallion from Totemhoof who signs up with an excavation team in the hopes of experiencing a life changing adventure. What he finds will send him into a head-on collision with forces he can't begin to comprehend- and, perhaps, a chance to fulfill his destiny.

Sequel to Antipodes- though you don't need to have read it to enjoy this, I hope.

AKA An2podes

Set in the far future after Celestia and Luna mysteriously vanished and the sun and moon froze in the sky, Antipodes is the story of two ponies thrust out into the harsh new world on an adventure to uncover what happened to the world so long ago.

(I wrote this when I was 17. it's pretty rough in places, but i think the basic ideas i had here were pretty cool. it's got a good skeleton)

Spanish translation here: https://www.deviantart.com/spaniard-kiwi/art/Antipodas-1-3-953524835

To save her sister and her faithful student, Celestia herself must travel into the ancient tunnels beneath the ruined castle in the Everfree Forest. She, and the guards who accompany her, must face the dark, twisting corridors. Still, what was Luna and Twilight's purpose at the old castle? Why were they unable to return of their own accord?

And what watches Celestia from beyond the light's edge?

On a dark night filled with even darker magics, an insane cult attempts to give Nightmare Moon a body and life of her own, utterly separate from Princess Luna. But, when the spell is interrupted by Celestia, something unexpected is created.

Now, a little black alicorn filly named Nyx finds herself living under the care of Twilight Sparkle, only to be haunted by memories and emotions from her past. Is she Nightmare Moon reborn or is she simply a doppelganger with a soul and mind of her own? Can Twilight Sparkle protect Nyx from those who refuse to see past her dragon eyes and black coat?

Or, will Nyx be forced to inherit the sins that may not even be hers and become the greatest evil Equestria has ever known?

This story is a sequel to Into The Depths

The horrors of the catacombs have turned to distant memories for Celestia, Twilight, and Luna. Discord’s change of heart and Twilight’s coronation have helped to ease and erase the mental scars from that harrowing night. The three princesses can even smile and laugh as Nightmare Night moves into full swing.

Yet, it should never be forgotten that those who have been touched by Tirek are forever within his grasp.