One thousand years is a long time to hold such responsibilities, to raise both the sun and the moon alongside one's royal obligations. Now, with the return of her sister, Celestia was looking forward to the reprieve the night could offer.
When the time came for the day to end and the night to begin, Celestia waited for Luna to arrive. Close to an hour passed, but Luna never came. Concerned, Celestia temporarily abandoned her duty to find her younger sister.
There was only one place in which she could be—her chambers. Celestia dismissed the guards before speaking through the wood. "Luna? It's time for you to raise the moon."
The responding voice was very small, despite the effort at the Royal Canterlot Voice, at sounding regal. "We cannot tonight. We apologize."
"What do you mean?" Celestia asked, stepping closer and almost pressing her ear to the door.
"We cannot," Luna repeated, a little louder.
Had they been any other pair, Celestia might have asked permission before entering her younger sister's room. Such courtesies, however, were often relaxed or ignored, and she quietly entered the chambers.
Finding the other princess was easy, even amid the silk that made a navy and silver silk hangings. Celestia parted them and joined her sister, who was resting on a pile of cushions, looking through the open doorframe, out past her balcony to the sky beyond and looking almost… forlorn. "What troubles you, little sister?" Celestia asked softly.
"We cannot raise the moon tonight," Luna repeated, not meeting the elder's eyes.
"But why can't you, little sister?" Celestia gently pressed. She had assumed that Luna would be eager to resume her duties, to embrace the freedom that came with them.
Luna hesitated; when she spoke, she didn't quite meet her sister's eye. "The moon is full tonight. Every full moon we…" She sighed, then dropped the effort at the traditional voice she'd only recently discovered was no longer tradition. "Every full moon I'm afraid that I'll become her again."
The mention of the thing that had overcome her sister so long ago made Celestia's heart skip a beat. "You won't become Nightmare Moon again, little sister," she reassured, nuzzling the younger alicorn.
Luna turned away, clenching her eyes shut and looking hurt. "You haven't felt the things I have felt, sister."
"What kind of things have you felt?" Celestia asked softly. Such a thing was not discussed lightly.
Abruptly, Luna stood, her next words directed to the open sky. "A tightness in my heart, like jaws. The way my heart burns, my head aches—" She turned to Celestia, her gaze tearful. "She's still inside me, big sister. Every full moon, I fear if I let her, she will take over again." Luna bowed her head in shame. "And I fear not even your most faithful students with the power of the Elements of Harmony may bring me back a second time."
The pain in her sister's voice was almost unbearable. Celestia stood and joined the younger alicorn, muzzling her trying to hold back her own tears. "I won't let it happen again, Luna. Not again—I promise."