Ting! Tang!
Celestia froze, turning as bells tinkled softly in the cavernous room. A gray unicorn stepped from concealing shadows, starry hat pulled low over his eyes.
“Starswirl! How—"
“This is a bad idea," he said grimly.
"It's all I can do!" Desperation strangled her voice.
"It is not." They matched stares, eyes hard. "She wouldn't thank you for this. You refuse to cut the root. Simplify. End it, Celestia."
"Do you know what you're asking?"
"Completely. You're strong enough. She could never match you, and the parasite hasn't changed that. How will she curse you, if others suffer in her place?"
"No, I…" Celestia turned, stained glass spilling chromatic wash across her coat. The gems before her sang softly from their pedestals.
"If you had pushed aside equivocation to act, your ponies would be safe. If you were decisive with Sombra—"
"If we had treated Sombra on his terms, we would be no better than him! This will not change that!" Her eyes flashed. "And you! You are out of place, where you should never be! How many jumps will you make, Chronomancer? Intruding, meddling, to sate your curiosity?"
"I'm unsure." Starswirl hesitated. "I'm not done yet. But this isn't about me."
"I will not kill my sister!" Celestia turned back to the Elements. "With these—"
"You can merely delay the problem. No prison will hold her; the crisis will return. They will not handle the parasite. Not for you."
"No." She grit her teeth. "No, there's more. My sister is still in there, still fighting."
His eyes narrowed, flashing. "Truly?"
In response Celestia seized his hoof, dragging him to the pedestals and shoving him at the blue gem. After a moment, he touched it. High, frantic whispers filled the room.
No, no, wait, help, please stop!
He yanked his hoof back, eyes wide.
"Truly. Help me. This can end with better than blood."
Starswirl sighed, rubbing his forehead.
"To stand against you now would simply ensure disaster. As you will, my Princess. Prepare; I will lure the Nightmare."
Celestia nodded, swiftly gathering the gems.
Starswirl began a spell, magic spinning from his horn in wisps and whorls. It moments, it was answered with stars.
"So, sister, I find you…" The vapor coalesced, firming into a mockery of Luna's beauty and elegance. "And you again! How?"
The fight was the work of moments. Starswirl fought desperately, standing his ground against a sea of midnight magic, even as Celestia struggled to sway the Elements. She finally brought them to bear, turning their rainbow power against her sister. There was a flash of brilliance, a rush of prismatic light as they crushed the Nightmare's form to vapor.
The corrupted alicorn screamed, her voice wrenching strangely as she disintegrated. A scrap of cloth fluttered to the ground, loosed from her horn.
Starswirl's eyes tracked the fragment, even as he fought to control his panting.
"No wonder you were so surprised." He limped forwards, hooves crunching on shattered glass. Out the empty windows the moon vaulted skywards. A burst of light touched it, imprinting a dark visage, locked in morose contemplation of the planet. He raised the dropped rag. It shone with stars, gold needlework brilliant. A single bell tinkled. He doffed his cap, matching the ripped cloth to an identical seam. "This is a duplicate. Another echo of me?"
Celestia nodded.
"She ripped my hat. Was I killed, then?"
"…I saw the body. You were old, older than I've seen you yet. Bearded." Celestia shakily lowered the stones, eyeing him uneasily.
"This… is going to put a crimp in my research." He stroked his chin, frowning.
"Surely you can avoid…?"
"I'm no longer sure changing the past is possible." He smiled wryly, looking to the moon. "No matter how I try. Now I've seen this… well." He shrugged. "It's set."
"Then whence your earlier desperation, your insistence?" Celestia carefully replaced the Elements on their pedestals, watching him closely.
"Ah." He sighed. "I simply can't not try. When I visit the past, it seems immutable. Yet when I try for the future, I’m smothered in possibility. I've viewed many outcomes from this night, how things resolve after my time, and… well, I'd wish for few of them. It seemed so simple, here, now, eliminate the cause. I thought it was weakness which stayed your hoof, that misplaced emotions kept you from justice. If I could sway you…”
“And I thought you knew me.” The Princess tried for a smile.
“I know you, Celestia.” He bowed. “I’ll never claim to understand you. Still, I didn’t expect to be quite so off. Yet this ending seems inevitable now." He closed his eyes, thinking. "Was it always so? If I hadn't come, would my absence have an effect?"
"That way lies paradox," she said wryly.
"Too true." He looked to her. "Perhaps I can leave you some small advantage, a guess at what will help.”
“Why bother?” Celestia sighed. “What can you change?”
“My future, eventually. It’s still in flux, and in time, we will share it.”
“Tell me, then.”
“You've delayed disaster by a millennium. After, your sister will return, still gripped by the parasite. If you can find the one marked with stars, she may aid Luna's escape from darkness. Otherwise, disaster will fall again, leaving you powerless to resist. Prepare desperately."
"Are you cryptic simply for the joy of it?" She frowned.
"Annoying, isn't it?" He grinned back. "No, I’ve got no more clues. Even these glimpses are tenuous. The future is a mess, much less orderly than the past. Time is strange, Princess." He turned to the moon. "Some say it heals, although I feel we simply leave our old selves behind, wounds and all. Who's to say death is any different? Sometimes I wonder how age will change me. Would I have recognized myself?" He looked at the scrap of bloodied cloth. "I wonder. I know it, now; this research will kill me. Yet stopping is no longer a choice. Even if I wish it."
“Do you?”
“At present, yes.” He smiled at her grimly. "But it seems that will change. When we meet, years and years from now, please don't pity me as I walk into this. Though some would say knowing your fate is a terrible curse, I love my work." He jammed his hat back on rakishly, and looked to the sky. "And to learn your sister may be redeemed… I will not count it a loss." He saluted Celestia. "Be well, my Princess."
There was a pop, a puff of air, and she was alone.