Looking up from her notes, Twilight Sparkle realized she wasn't hearing the music from Pinkie Pie's "Hooray for Friday" party down the street anymore. Had it gotten that late?
She yawned, stretched, stood, and shook herself. She'd stopped by the party earlier, of course, had said hello to everypony and had some punch and cake, but this underwater breathing spell she was working on just kept nagging at her; she'd said her 'good nights' pretty early and had come back to the library to get another few hours of research in before hitting the hay. But now, walking into the foyer where the big grandfather clock stood quietly ticking away, she realized she'd completely lost track of the time.
Three soft clangs from the clock, and she had to give a little laugh. Good thing she didn't have anywhere she had to be on Saturday, or--
A knock sounded at the library door, gentle but definitely the tap-tap-tap of somepony's hoof. Twilight blinked at the door, looked back at the clock, thought briefly about waking Spike, but no. This was Ponyville. Even at three o'clock in the morning, it wasn't like there was anything here she really had to worry about...
Another few taps. Twilight set her horn to glowing, used the power to nudge the handle down and push the door open.
Darkness outside, the stars glinting through the cold of late autumn over the town square--and something darker, something like a piece of blackest night come to life, a mane and tail flowing, wings outstretched, a horn jabbing upwards and making Twilight gasp, "Nightmare Moon!"
"Ummm," came a slight voice, and a figure stepped into the light thrown over the doorstep from the library's main room, a winged unicorn only slightly taller than Twilight herself, her deep purple mane falling over one eye. "I don't use that name anymore, actually."
Her cheeks heating with embarrassment, Twilight bowed to the ground. "Princess Luna! I'm so sorry!" She glanced up at the pony princess, so much like a shadow cast by her older sister, Twilight's teacher, Princess Celestia. "I...I've been buried in my books the past however many hours, so my mind's a thousand miles away!"
Princess Luna shrugged and didn't meet Twilight's gaze. "It's a natural enough mistake," she murmured.
Some silence, then Twilight shook herself again. "Please, Princess, won't you come in?"
"I--" She looked back at the dark and silent town, looked up at the stars, bit her lower lip, pawed a hoof at the ground; Twilight couldn't imagine what was making the co-ruler of all Equestria so nervous, but the scent wafting from the princess definitely had more than a little fear in it. "Yes," she finally said, bowing her head with an air of defeat that confused Twilight even more. "Thank you."
Twilight stepped back, and Princess Luna glided into the foyer as gracefully as a cloud across the face of the moon, the darkness of her flanks taking on a silky sheen in the warm glow of the library's lights. All too aware of the state she'd left the main room in--whenever she began researching a spell, it seemed like every book in the place held a little bit of what she needed--Twilight stammered out her standard apology: "I'm sorry about the mess, your Highness. Like I said, I was--"
"Yes." The princess smiled tightly over her shoulder. "So it's I who should apologize to you for interrupting."
"Oh, no, Highness! Never!" Twilight moved to the princess's side and gestured with a hoof into the library. "I hope you know that you're always welcome here!"
The taller pony blinked her big dark eyes down at her. "You sound so sincere, I could almost think you mean it." She looked away and trotted in.
Her confusion growing, Twilight stood for a moment, then rushed after the princess, activated her horn, and swept the books from part of a table. "Can I get you anything? Tea? A sandwich? I might have some cake from--"
"Nothing, thank you." The princess settled at the table, and her head drooped again. "I don't want to be any more trouble than I already have been, than I already am." She sighed and added, "Than I already am going to be..."
"Trouble?" Twilight suddenly felt wide awake. "I...I don't understand, your Highness."
Nothing for a long moment, Princess Luna not raising her head, a chill spreading down Twilight's spine. "If anyone in this world knows me, it's you," the princess said at last, her voice as quiet as a winter midnight. "Sister Celestia tells me you alone among all our subjects learned of my prophesied return, that you alone had the understanding to stop me when I...when I..."
Her voice trailed off, then she looked up, a sudden determination in her face. "Celestia and I have vowed that what happened between us a thousand years ago will not happen again, and therefore we...she...I-- Oh, blast it all!" The princess leapt to her hoofs, her wings spreading like dusk after a long day and raising her into the air, a dark glow springing up around her horn. "This is crazy! And stupid! I shouldn't've come here! There's nothing you or anypony can do to help me!"
"To help you?" Twilight repeated. "Princess, if you're in trouble, just tell me, and I'll try to--"
"Stop." The coldness in the word made Twilight's ears fold back. "Don't offer what you can't deliver."
"Excuse me?" Whether it was the lateness of the hour or the little frustrations of her spell research, something inside Twilight snapped, and she glared up at the younger of Equestria's two immortal rulers. "Maybe if you'd just tell me what you're talking about, I'd have a better idea what's going on around here!" Princess Luna's dark eyes narrowed, and Twilight wanted to sink into the floor. "If you wouldn't mind, I mean, your Highness."
The slow strokes of the princess's wings made the only sound in the library for a moment, then she sighed and settled to the floor once more. "Celestia's decided," she began, her gaze focused several feet off to Twilight's left, "that is, she and I have talked about how I...what I did and how I changed and why and...and-- Blast it all!"
Twilight could only stare as the princess shook her mane away from her face, those dark, dark eyes more intense than ever. "Celestia is going to give me all her authority for one week so that I can rule over both night and day the way she had to for the past thousand years." The ghost of a smile pulled her snout. "She says she's earned a vacation." The smile vanished. "But it's really her way of trying to stop me from going insane again, her way of showing me that we're truly equals when it comes to Equestria, her way of saying that she trusts me and loves me and wants to help me."
Twilight felt her jaw drop, but Princess Luna was going on: "I just don't know that she should trust me. I mean--" Her breathing was getting shakier and shakier. "When I was the Mare in the Moon, exiled up there by myself for all those centuries, I used to rant and scream and swear up and down that I would bring eternal night upon Equestria once I returned! And now?? Now my sister is turning over the reins of power to me!" She stomped a hoof with a crack like a thunder clap. "Like I never lost my mind! Like I never tried to destroy everything we'd built together! Like I was never Nightmare Moon at all!"
In the silence, all Twilight could think to say was: "I thought you didn't use that name anymore."
Princess Luna coughed something that could have been a laugh. "You’re exactly right." She took a stance on the library carpet, dark power surging through her mane. "Since I can't seem to talk Celestia out of doing this, I've decided I need you there in Canterlot with me, you and your five friends who stopped me before." Those eyes fixed on Twilight, and she felt them sift straight down to her soul. "So, Twilight Sparkle, I ask you: summon the Elements of Harmony, attend me next week in Canterlot, and be prepared to strike me down should I once again become Nightmare Moon."