Finally, after much recounting and reliving your moments of awesomeness, you arrive at what you're pretty confident is the right answer. Probably. Assuming you didn't miss any secret passageways or a set of stairs to a whole extra basement level.
You spin the dial to the number you've settled on and press the button in the center before you can get any less sure. It sinks into the door at your touch. Something starts to click as hidden gears begin to move and make the decorations across the door spin and contract. Monsters on either side of the center divide pull their claws back from the seam.
The clicking stops, the movement freezes, and...
...the doors swing open wide!
There's... silence.
The highest room in the tower is empty. No decorations, no applause, nothing. Just a circular room of bare rock and wood without so much as a scrap of decoration. The only feature even worth mentioning it a big hole in the wall with a small exit sign above it.
Is this really it? This is the end? It's kind of... anticlimactic. Maybe there's something wrong with the spells that happened when you ran the course but that Starlight and Trixie missed.
Better go tell them then. You spread your wings and take off, circling around the tower a few times to bleed off height in an easy glide. You come to a landing by the entrance right in front of Starlight, who looks surprised to see you.
"Dash? What are you doing back so soon?"
"I think I found a broken spell or something. One of the rooms is just empty with nothing in it."
She frowns. "That's not good. Which room?"
"The one at the very top of the tower."
"The one at the—that's the winner's room! How did you get there so fast? You can't have done the whole house already!"
That sounds like you may have set a record. "You might not have heard, but I'm very fast."
"Not that fast." She squints at you suspiciously. "I know the optimal route and even then it takes me at least twenty minutes to collect everything to get past the tests."
You shrug. Guess some ponies are just sore losers about having their records broken. You never pegged Starlight as one, though. "Like I said, I'm—"
"A Cheater!"
You spin around to find Trixie standing in the doorway of the mansion, her face set in a glare and her hoof pointing at you accusingly. Gone is her vampire costume, replaced with some kind of Discord-themed suit. Her fake wings made of magic twitch and bounce with very real frustration as she stomps over and pokes you in the chest. "I don't know how you did it, but you cheated!"
You feel your anger rise at the accusation. You? A cheater? The very thought of it is laughable. Sure, you play to win but there's not much point in winning if you cheat. "I did not!"
"Oh yes you did! I was watching from the control room. You went straight from the foyer to the exit. No hesitation, no side trips. You didn't even get stuck by the challenge questions even though there's no possible way you could have known the answer!"
"Maybe I'm just lucky!" Though... how did you know the right answers? You felt confident at the time but...
"Or maybe Trixie's onto something." Starlight's voice is cold and calm. You notice her horn is lit with a spell and there's green flecks of light dancing over you. "So. Rainbow Dash. Care to tell me why I'm detecting traces of time magic on you?"
Time magic? "The hay? I don't know."
"I do!" Trixie declares. " I bet you have an ancient and powerful magical artifact that lets you turn back time and make different decisions. You try every path but we only see the one you want us to see!"
That's ridiculous. "That's ridiculous. Do you see an artifact on me anywhere? I'm not even wearing a costume!"
"Maybe this isn't your frst time being caught and you knew to stash it first."
"There's no artifact!"
"Dash, are you feeling okay?" Why does Starlight sound so concerned suddenly? Wasn't she angry too a second ago? But you feel fine.
"I feel fine."
"No headaches? Strange urges? Random blackouts or moments of forgetfulness?"
No, you haven't felt anything like that. "No, nothing like that." Before you can ask for more detail, Trixie derails the conversation again.
"A silent partner then. Ah ha! Twilight Sparkle! She used Starswirl's time spell to loop Nightmare Night for you so you could make all my hard work on the haunted house look worthless."
You scoff. "Twilight wouldn't do something like that."
"Some other unicorn then! There are many jealous of my magical skills."
"Oh would you just drop it already? I didn't cheat and I don't have any time magic on me!"
"...She's right."
Finally! Validation from Starlight. What is up with her today? First she's angry, then concerned, now she just looks coldly determined.
"Hold her!" she suddenly cries.
You freeze in place as your body is sealed by Trixie's magic. You try to shout and protest, but even your jaw is solid as a rock. Starlight walks around so your frozen eyes can see her again. There's a fierceness to her features that you're not used to seeing.
"So she was lying," Trixie declares even as she grunts with the effort of holding you still. "I knew it."
"No. She wasn't lying. And that's not time magic I detected either." She looks deep into your eyes like they're some kind of telescope. "It's dimensional magic."
She lights her horn with a spell, a powerful one too if the massive corona means anything. "Don't worry. You won't feel a thing." Wait, she's casting it on you? No! You struggle against the magic but you can't move an inch! Can't dodge or anything as Starlight lowers her horn and fires the massive spell right at—!
You feel... warm. There's a strange sensation of disconnection, of difference. Like you've been walking around with a cast on and sudd—
"Is she okay?"
"Should be. The spell shouldn't have affected her at all."
"Perhaps she's just tired?"
"Ah reckon Ah would be."
"Dashie! Wake up!"
"Pinkie! That's not how you rouse a pony from unconsciousness."
"Oh! Right. We need smelling salts!"
"Why did you have those?"
"Just in case!"
A sharp tang of salt woke Rainbow Dash from the depths of slumber. The first thing she noticed was how strange she felt. Her whole body felt lighter. Freer. Like she'd taken off a weighted training uniform she didn't know she was wearing.
The second thing she noticed was the faces of all her friends clustered around her.
"You're okay!" Pinkie cried as she leapt forward and snatched up her her friend in a bone-crushing hug. "I was so worried."
"Pinkie... can't... breathe!"
"Yes I can. Oh, you mean you can't! Sorry Dashie!" She released her with an apologetic smile.
Twilight stepped around her and passed Rainbow Dash a bottle of water. The pegasus took it with a nod of thanks and gulped it down. "We came as soon as we got Starlight's message," she explained. "How do you feel? Any lingering compulsions?"
Dash shook her head. "I'm good. I feel like I just ran an Iron Pony, but nothing hurts. What happened?"
Starlight stepped forward. "It's hard to put it in non-academic terms, but you were astrally connected—"
"You had a worm in your brain!" Pinkie jumped in.
"I had a what?!"
"A parasite, Pinkie," Twilight corrected. "Not a worm."
"Even that's technically wrong." Starlight countered, "since it didn't feed on anything."
"Will somepony please just tell me what was in my head!"
Starlight seemed unsure for a moment before reaching back into a bag at her side. "This." She held up a jar containing... a thing. It defied description as much as it defied her ability to look at it properly. It had colors, but none she could name. It was like a crack in the air, but it also moved like water and like a wind-blown string.
"At some point—I don't know how or when—you picked up a transdimensional hitchhiker. Or were chosen by it. Again, I don't know. Either way, it's been puppeteering you around like a marionette." She tapped the glass, making the thing inside writhe.
Rainbow Dash could only stare, gobsmacked, at the caged anomaly. "That thing was in my head?"
"Not literally, no, but dimensionally, yes."
"But I don't remember feeling weird or having to fight for control of my body or anything."
It was Twilight who shook her head in response. "You wouldn't have. It sat in the metaphysical conception of your mind and made decisions on your behalf—what to say, where to go, how you felt—and your body put them into action. Your conscious mind had no say and probably just rationalized away until it convinced itself that the choices must have been yours."
"Oh. Wow, that's... geez. And I had no idea it was happening." She went quiet for a minute. "Can I squish it? It'd like to squash it like the bug it is."
"Unfortunately, no," Starlight answered. "It's not made of real matter. Besides, this isn't its true body. Just a portion of it extended into our reality. But while we've been talking I've been forming a dimensional tunnel and tracing its connection back to the source. And I think—" the corona around her horn changed color "—I've almost reached its home realm."
Suddenly, her horn started to crackle with black and white sparks.
"Almost there!" she grunted. "There's some kind of barrier. Twilight! I need a boost!"
"I'm here!" Her magic poured into Starlight, and from Starlight into the caged entity. It surged along the tether that connected it between dimensions like a raging tsunami, following the path back up the chains of dimensions until—