The crystal chime of the castle’s nightbell roused Twilight Sparkle from the pages of slumber. She marked her pillow with a phoenix quill bookmark, stood, and trotted through the castle’s long halls to the foyer.

“Good morning!” she said, opening the castle’s cover. “How can I—Oh, it’s you.”

“Uh, hey.” The Nightmare ground her hoof into the dirt and looked anywhere but Twilight’s face. She seemed out of place, standing in the sun. Diminished by it, little more than an inky afterimage on Twilight’s retina, shifting and dancing and refusing focus. “Look, this is gonna sound kinda weird, but would it be okay if maybe I stayed with you for a few days? Luna kicked me out.”

“Good.” Twilight tried to slam the door in the Nightmare’s face, but it had melted and fused with the floor again. She scowled and made a mental note to buy more crystal carpet cleaner. “About time. She finally got sick of your cheating?”

The Nightmare winced. She scratched at the side of her neck and mumbled something. She was naked, Twilight noticed, bereft of her usual regalia. Presumably all left behind in her flight from Luna’s wrath.

“I ought to say no,” Twilight said. “But my castle’s broken, so I guess you can stay. You know where the guest room is?”

“Actually, I was hoping I could share your—”

“The guest room,” Twilight repeated. “Or back to Luna. Your call.”

The Nightmare shivered at that. “The guest room is fine. Can I eat your brain real quick?”

“Sure, sure.” Twilight braced herself. This part always stung like a—

Twilight woke with a start. The blanket had tangled around her legs, binding them, and her sweat soaked the sheets beneath her. Not even a hint of the dawn encroached upon the nightscape outside her window.

She groaned quietly as the dream settled into her memory. Its edges were sharper than a dream’s should be, as real as darkness smothering her room. She let out a shaky breath.

“Dammit, not again.”

“So, it’s like, haunting the castle?” Rainbow Dash asked. She peered around at the shadows.

“Not the castle. Me.” Twilight took a sip from her chamomile. “Again.”

“Wait, this ain’t the first time?” Applejack asked.

“No, Luna loses track of it once a year or so. Usually it runs off to one of the other princesses, then we have to hold it for her until she can take it back. Last year Cadence had it for a week.”

“Well, that sounds lovely,” Rarity said. “What do we do about it?”

“Nothing,” Twilight said. “It’s harmless. As long as you don’t give into its temptations.”

The Nightmare was waiting in Twilight’s bed when she retired for the evening.

Twilight frowned. “I said the guest room.”

The Nightmare ignored her. She stretched atop the sheets, expressing the graceful contours of her chest and belly and legs, somehow contriving to reveal everything and nothing at the same time. It seemed impossible for such long limbs to even fit on her bed, but there she was.

Twilight tore her eyes from the sight and hopped up beside her. This was her bed, dammit. She wasn’t going to let some two-bit imaginary hussy chase her from it.

“Hey,” the Nightmare whispered. “I’ve been waiting for you.”

“I bet. Go back to your room, or go to sleep. I don’t care which.”

“Liar.” The Nightmare snuggled up behind her. In the darkness, at night, she was far larger than Twilight. Long legs curled around Twilight’s body, hugging her close against the Nightmare’s chest. Her coat was cool against Twilight’s back.

“Trying to sleep here.”

“Silly, you’re already asleep.”

Oh. That explained why they were lying tangled together on a giant book floating in the sky above Ponyville. Twilight frowned.

“Bit for your thoughts,” the Nightmare whispered. Her voice was filled with spiders.

Twilight rolled over in the demon’s grasp. “What do you want?”

“You. Luna is so boring these days. But you? We could rule the world, Twilight Sparkle.”

Twilight snorted. “Try offering me something I want, instead.”

“Something Twilight Sparkle wants?” A wide grin exposed teeth like stars. “Knowledge, perhaps? Forbidden knowledge?”

That had a dangerous sound to it. Twilight tried to push herself away, but the Nightmare’s grip was too strong.

She stopped struggling when it became clear how futile it was. “There’s no such thing as forbidden knowledge. Anything can be used or misused. What matters is the pony.”

“So naive. Your youth is delicious, Twilight. I could drink it all night.” The Nightmare pressed her muzzle up under Twilight’s chin and inhaled deeply. “You think everything is in your books, don’t you? Everything worth knowing, locked away on those pages. Safe and harmless.”

“The written word is the greatest invention in pony history,” Twilight said. “You’re a monster. A parasite in my dreams. Don’t pretend there’s anything you can teach me.”

“Pretend? No, no need for that.” the Nightmare whispered. She released Twilight and spread her legs, exposing every forbidden bit of her to Twilight’s hungry eyes. A seam appeared in her flesh, from groin to chin, and her body opened like a book. Lines of black text covered her organs and bones and all the spaces in between. The scent of hot iron filled the room.

Twilight read all the night long. As always, the truth was in the margins.