It was not like Celestia to be so… Informal… Twilight was worried, Celestia simply asked her to arrive at Canterlot Castle as soon as possible, something had the princess worried, and that sent a chill up Twilight’s spine. “Spike!” She shouted out, trotting into the sun-lit hall of her Canterlot home.

“Yeah?” The baby dragon said, pulling himself up from the pile of cushions he was lying on, right underneath the enchanted hourglass, “What did the letter say?”

“N-Nothing…” Twilight stuttered, “Celestia only demanded that we come to the Castle this instant!”

“Well, that can’t be good…” Spike said, half-curled, slowly pulling himself up to his clawed feet.

“Exactly! I… I don’t know what to do, should I go, of course I should go… But, what has her so worried, should I prepare, but I don’t know what to prepare for… Ohhh, is it some kind of emergency, why would she need my help, I-”

“Twi! Focus…” Spike shouted, breaking Twilight from her spell of paranoid mania.

“Y-You’re right, I-” At that instant, there was a flash of golden light, and Spike gasped as Twilight vanished before his eyes.

“Ah…” Spike paused… Then, in a blink of an eye, he was suddenly standing before Celestia, her golden throne sparkling in the dawn light filtering through the stained glass, “How did~”

“Princess Celestia!” Twilight gave an ear-piercing shout, “Forgive me for not coming right away, what’s wrong! What’s going on!” Twilight continued to shout, hyperventilating as she bowed before the sole monarch.

“Shhhh~ Calm yourself,” Celestia said, but Twilight wasn’t listening, her mind gripped with panic… Celestia rolled her eyes, then, channelling a small pulse of magic through her horn, her student instantly calmed, going completely still for a second or two, before collapsing to the floor and springing back up the next second.

“What was-”

“Just a small stunning spell, don’t worry, I just needed you calm and focused…”

“T-Thank you, Princess… But why am I here…”

“That was what I’ve been trying to get to, sorry for teleporting you in out of nowhere, but there’s a problem... “ She paused, sighing to herself, and whispering too quite for her student to hear, “This was not the test I had planned… Forgive me…” Then, regaining some composure, she stood up, walking down from her throne, “Come, Twilight, follow me…”

As they began trotting down the marble and gold halls, Twilight noticed that not a single royal guard was present, and the unicorn gave a look of concern and confusion to her teacher. After a few minutes of thought, Celestia began talking, “Twilight… Do you know the story of the ‘Mare in the Moon’”

“Yes, something about…” Twilight paused for a second, “Princess Luna... Correct?”

Celestia sighed again, “Yes, my sister… She betrayed Equestria, and I was forced to banish her body, and imprison her soul in the moon… But there is more to it than that… What do you know about dream magic… Have you ever read M. Neigh’s book about the occult?”

“Well, regarding the first question, not much I’m afraid, isn’t most dream magic in the restricted section of Canterlot’s library?”

“Yes… But just in general, I’m not talking about spells, just theory and methodology…”

“Oh… Well, I know it has to do with interfacing oneself with the dream realm, and that the dream realm is a magical pseudo-space underlying this reality, similar to the theoretical ‘Limbo’... I think it’s also mentioned that part of the magical current within the dream realm can take on consciousness?”

“Yes, the Tantibus and Ursa, but they aren’t important for what I’m talking about… My sister, Luna, was quite versed in dream magic, it was her talent.”

“Yes, so?”

“She knew that the dream realm was a portal into the minds and souls of every living creature on this planet, a realm where time and space meant little… There were plenty of ponies who believed that the dream realm was in some way divine, plenty of ponies who worshipped dreams, and the entities within, do you know of them?”

“Yes princess, vaguely, I don’t know all that much I’m afraid…”

Celestia was silent for a minute or so, then spoke up, quietly, “When I banished my sister… There was something… Different about her… Something darker, a blackness, a living blackness within her, and I knew that whatever that thing was, it wasn’t my sister. In the thousand years since I’ve been doing research, and I think, my sister encountered something in the dream realm… And I have proof…”

Twilight shivered at Celestia’s suddenly dark tone, and stood up a bit straighter, “W-What is it…”

Celestia was silent, simply guiding her student through the castle, through places Twilight had never seen before… Eventually, they entered a small room filled with tomes, scrolls, and a single glass sphere atop a dais. There was a magical pressure in the room, strongest at the center, “Twilight…” Celestia spoke up, “A century ago, I found… For lack of a better word… A cult, within the town of Hollow Shades… They were worshipping something, a darkness, something that I could feel growing stronger within the tides of the dream realm. I recognized their dark magic, it was the same power I felt while combatting my sister… I, killed, the cult…”

Twilight gasped, she knew Celestia was a great warrior in the past, but to actually hear that her mentor had… Killed another living pony… Shocked her, but Celestia continued, “I had to, I had to seal away what evil they were summoning into this realm, I made a sphere of enchanted crystal and channelled the darkness into it, and placed it under many arcane shields and wards to keep that… Thing trapped within.”

Twilight’s eyes moved to the crystal sphere in the middle of the room, something was swirling inside, Twilight could almost see a forest, large trees, and rabbits walking through the woods, and… Something, a presence within that woods, no, above the woods, like something looking down on the world from above. “You can feel it, can’t you,” Celestia said, her tone icy cold, “I made a world inside that sphere, a fake reality to fool the creature, but I feel my spells weakening… That thing is growing closer to escaping its glass prison every day.”

“W-What do you want me to do?” Twilight asked.

“I need someone to go inside that world, find the creature, and slay it…”

Twilight was silent… “Why me?” She finally asked, “Can’t you send one of your battlemages, or some other powerful wizard?”

“Because I trust you, Twilight…” Celestia said. “You’re the only one who I know would be able to get this done, if you fail…”

“I know…” Twilight replied. She sighed, “Alright, send me in…”

“Now?”

“Yes, if this thing is as dangerous as you say, I need to be as quick as possible…”

Celestia nodded, “Alright… Stand still.”

Twilight nodded as she stood up straight, staring deeply into the sphere, it was some kind of magical terrarium, a world within a jewel, the magical energies flowing through it was impressive… Then, her body slowly went numb, she could feel the power of her master flowing over her, intertwined with the sphere… She closed her eyes, and a minute later, she opened them to the sound of birdsongs. All around her were the woods she had seen, large open grasslands dotted with groves of trees, tall trees, with round tops and little bushy puffs of leaves growing off their branches.

Author's Note

A Terraria Crossover... I'll be uploading this for the time being... Not really into RWBY right now, so don't expect any new chapters for Fillies and Monsters for a while.
PS. You may have noticed, but this takes place before the first episode of MLP