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Celestia trotted back and forth in her room. This was intolerable! She had ruled all Equestria by herself for a thousand years, faced down dragons with perfect serenity, but this, this preyed on her mind, an itch she couldn't scratch, a burr between her wings she couldn't reach!
"Trouble sleeping, sister?" Luna's voice asked. Celestia whirled. Standing on the balcony was possibly the worst pony in the world for her to try to explain this to. "Would a glass of warm milk help?"
Oh, no. And she was feeling playful, too. Celestia withheld a groan. "No, thank you, Luna. I will be just fine."
Luna cheerfully stepped off the balcony, into the room. "Oh, sister, I would hardly be doing my duty as guardian of the night if I were to let any pony, even the Princess of the Day, be so terribly distraught without at least attempting to render some assistance. Please, tell me of your troubles."
Celestia sighed. "You know my trouble, Luna. And I know how much it amuses you."
Luna nuzzled her with a mix of genuine sympathy and the sort of gleeful malice only a sibling can truly offer. "Why, sister, every pony in Equestia adores you. How could you possibly have any troubles?"
Celestia nuzzled her back anyhow. She was such a twit right after the New Moon. "Every pony, yes," she said.
"Not all of them quite as much as others," Luna mused aloud.
Celestia rolled her eyes. "Pip is a wonderful, loving little foal, and adorable beyond measure, yes."
Luna spread a wing across Celestia's back. "Oh, indeed he is! Do not tell me that such a cute little colt could cause the ruler of Equestria such heartache!"
Celestia shook her head. "You know the trouble, Luna."
Luna theatrically raised a hoof to her chin. "But if every pony in Equestria loves Princess Celestia, how could there be anything to trouble her?" She gasped. "Oh! A non-pony! Has Spike the Dragon so perfectly supplanted you in his heart with the lady Rarity? Perhaps Tom dreams of a shiny little piece of quartz!"
Celestia blinked. "... Do rocks actually..."
Luna's giggle was really tremendously annoying. And she didn't even answer the question. "Oh! Bellerophon Michaelides, is that the problem?"
"You know he is," Celestia said with all the patience she could muster.
"But sister, he gives you all your due respect, and he has been a model citizen in almost every way!" Luna mock-protested. "How could he be any sort of a problem?"
"I wronged him, sister, and he cannot accept my reparations," Celestia said, and turned to sigh into the fire. "As you well know. And beyond that, he is dangerous. More dangerous than he knows. There are still times, even now, even knowing him, when I remember that perhaps I should carry out those threats I leveled upon him. Twilight would never forgive me, Rainbow Dash would actively lead a rebellion against the Tyrant Sun, Lyra Heartstrings would slay her way to the throne room and force me to destroy her as well, and yet. What he might do."
"Might he seek to conquer Equestria?" Luna asked softly, still teasing, but with darkness in her voice. "Might every pony die because of him?"
"They might," Celestia answered quietly. "If he did what even Nightmare Moon was unwilling to do to her sister. As you know well. We are neither of us safe while he lives."
Luna's wing rubbed against Celestia's back. And then Luna's eyes widened. Her wing fell from Celestia's back as the dark alicorn dropped to her rump, laughing like a maniac.
"At last! At last! At last the truth comes out!" she choked out. "Thou dost not fear him! Thou... thou..." She fell to her side and pounded on the floor with her hoof, gasping out gales of laughter, and then sing-songed out, "Tia wants Leeeero, Tia wants Leeeeero!"
Celestia felt as if the sun had risen on her face instead of the sky. "Luna!" she snapped. "I am not joking!"
Tears ran down Luna's muzzle. "Thou art fooling thyself, sister!" she panted out. "Thou dost not fear him, and if thou wert honest with thyself, thou wouldst admit it! Thou... thou..." She broke into giggles again, then gasped, "Thou art excited by him! The touch of danger! The thrill of the alien!"
Celestia stared into her fireplace. It lit. That was what it was there for, after all.
To let her know when she was getting angry enough to set things on fire by looking at them, that is.
She loved her sister. Really she did. But could anything be worse than having a sister who knew what she dreamed about?