So, Saturday's new episode of MLP:FiM was leaked today… I am having the worst case of Deja Mustard right now.
Anyway, here's the lowdown from TV Tropes, along with your chance to watch the episode below. If you wanna wait until Saturdy, I don't blame you.
Twilight Sparkle has combed through every last book in Ponyville, looking for any information she can find on the mysterious chest from the Tree of Harmony and the keys to unlock it. Right when she's about to lose hope, Spike belches up a letter from Princess Celestia directing her to the ancient ruins of her and Luna's former castle in the Everfree Forest to look for a book that could help with her research. Spike would much rather stay out of the creepy castle, but Twilight is so excited that she pays no heed to their spooky surroundings.Meanwhile, Applejack and Rainbow Dash's latest dare contest has come down to a tie. After their scorekeeper Pinkie Pie leaves for her new job as Ponyville's new school bell ringer, Applejack comes up with a tiebreaker challenge — in the ancient castle. According to legend, before Nightmare Moon was banished, a bit of her dark magic was left behind to haunt the castle halls at night as... the Pony of Shadows!
The girls decide that the one who can stay in the ruin the longest without being scared away will be winner.As this
goes down, Rarity brings Fluttershy and Angel to help her collect some old, reparable tapestries from — you guessed it — the castle. While fidgeting behind a giant drape, Fluttershy stumbles upon a rotating wall and winds up right in front of Applejack and Rainbow Dash, scaring them deep into the castle halls with her muffled cries. The wall spins back around, and Fluttershy and Rarity agree to take care not to trigger any more traps, only to fall straight down a trapdoor accidentally opened by Angel. As the two frightened pairs continue uncovering secret doors and hallways, Angel finds Twilight and Spike reading in the library, startling Spike into activating another secret passage leading to a room housing the book from Celestia's letter: her old journal. Twilight dives nosefirst into the book, eager to know what secrets it may hold. One of these secrets is an ominous pipe organ, which the ponies begin to hear being played by a mysterious cloaked figure. With each key pressed, the figure activates even more traps that Applejack and Rarity fall into, separating the girls.Alone scared out of their wits, the four quickly find themselves backing into the same room and bumping into each other, sending them into a panic. Their clamoring alerts a nearby Twilight, who magically freezes them all and calms them down. Suddenly, the girls hear the ominous sound of the pipe organ and follow it to find what they think is the Pony of Shadows. Since Twilight has read up on all the castle's secrets from Celestia's journal, however, she is unafraid and approaches the figure and uncloaks it to reveal...Pinkie Pie!
It turns out that after Pinkie finished her bell ringing job, she went into the Everfree Forest to pick some bluebells for a party, only to spot her friends going into the ancient castle for a "party" of their own, so she decided to help by playing some music. As her friends finally relax, Twilight admits that while she couldn't find anything about the chest, she's glad to have helped her friends. Inspired by Celestia's journal, Twilight proposes they keep a journal of their own so they can keep track of what they've learned. The girls laugh off the tale of the Pony of Shadows into the night, never once noticing the eerie, glowing-eyed shadow looming outside the room...Watch here, if you dare.