The debate over the Amulet of Aurora lasted until dusk. Princesses Celestia and Twilight agreed they ought to use it to summon a tsunami and wash their enemies out of their lair before any more of Equestria was overtaken. Princess Cadence and especially Princess Luna, however, were in opposition. As Watchkeeper of Dreams, Luna understood better than anypony how the psychological damage to the evacuating San Franciscolt citizens could scar and embitter them, and she argued this point hard against her sister and Twilight.
“It is not our place as Princesses to force our subjects into sacrificing their needs for our interests. Even if we think our interests are best for them. We may be rulers of Equestria, but we must not allow ourselves to be tyrants. I learned this lesson the hard way, sister, and I would do everything in my power to ensure you do not take upon yourself the same agonies I bore when I became Nightmare Moon.”
“Thank you, Luna.” Celestia wrapped her sister in a hug. “And I would do everything in my power to ensure you do not continue to suffer from your memories.” She retracted her arm and looked her sister in the eyes. “But the threat to Equestria is just too great. Believe me, I do not wish to exercise this forbidden power, but I hold the needs of my people above even my own moral misgivings.”
Luna scowled and turned away.
“I’ve got an idea!” Twilight interjected. “What if we summoned a tsunami, but at the same time protected the city from it? We only want the waters to flood the caves, after all. Maybe we can modify a water-deflection spell to amplify it to city-scale!”
“An excellent suggestion, Twilight! Let us visit the library and work together. We may be able to finish such a spell by the morning! Luna, Cadence, will you join us?”
Princess Cadence yawned. “I should probably go check on Flurry Heart and relieve the poor babysitter.”
“I have my night duties to perform,” Luna stated flatly. She stalked out toward the balcony where she typically did her work without meeting her sister’s eyes.
Celestia turned back to Twilight. “Looks like it’s just the two of us. Shall we?”
“Amazing. How do you do that?”
“Research is a skill, Princess, and I have a lot of practice.” Twilight didn’t look up from the three books she was flipping through simultaneously while also jotting notes on a parchment. “I’m finding several spells that can widen the sphere of influence of other spells, but none of them correct for dilution of potency. Do you know of any books that mention magnification of spell efficacy in addition to casting radius?”
“...I’ll go have a look.” Celestia started to walk away down the rows of shelves, but suddenly paused and glanced over her shoulder. “Would what you need would be under ‘P’ for ‘Potency’, or ‘E’ for—”
Celestia noticed an odd green glint in the eyes of the librarian pony levitating a glossy object above Twilight’s head.
“Watch out!” She exclaimed, throwing herself into Twilight and swatting the object out of the unicorn’s magical aura.
“HISSSSSSS, you fools!” The librarian transformed into Queen Chrysalis and quickly retreated into the labyrinthine shadows of the dark library.
“What was that?” Twilight was wide-eyed and shocked, lying in a pile of the books she had just been reading.
“I don’t know...” Celestia began to slowly walk toward the object Chrysalis had been levitating, now lying on the floor. She picked it up in her hooves and contemplated it. “It is… The Helm of Shadows.”
“Princess, please put that down!” Twilight pleaded, now understanding the danger they were in, but Celestia appeared transfixed on the Helm and did not acknowledge.
“What is going on in here?” The commotion had alerted Princess Luna and she burst into the library. When she saw her sister slowly lifting the Helm of Shadows above her own head, Luna’s mouth fell open and all she could do was stiffen and whisper “no.”
Celestia’s grin stretched unusually wide as she began to slide the Helm onto her head.
“No!” Luna now shouted, and dove for her sister. She managed to magically wrest the artifact from Celestia’s grasp and retreated a few steps to make sense of the situation.
Twilight was frozen in place, her face wearing pure, dumb shock.
There was a movement in the shadows that looked like Queen Chrysalis, horn powered to prepare to take the Helm again.
And Celestia was stalking toward Luna. Her pupils were narrow with greed and her face was contorted to express a depth of rage Luna didn’t know her sister could feel. It took her one moment to understand that Chrysalis tried to use the Helm to recruit another ally, and that Celestia had accidentally touched the artifact in the struggle.
It took Luna another moment to see that having touched the Helm, Celestia was already consumed beyond herself by the lust of its dark power.
It took Luna a third moment to process that there was only one way she could save her sister from the shadows. She felt all the worst parts of herself rising up inside her, choking her. Darkness, jealousy, anger, deceit, hate, greed, all coalesced into a face that still plagued Luna in her nightmares. Then the corrupted face transformed into Celestia’s, and the pain of the future seemed harder to bear than the pain of the past.
“Twilight,” Luna urged, “you have defeated me once before. I know you can do it again!” Then she looked into Celestia’s angry, delirious face, and tears sprang into her eyes. “Forgive me, sister.”
Celestia’s pounce was quick but Luna was quicker. The Helm of Shadows was reunited with its original bearer, and thus it released Celestia from its influence.
Princess Luna began writhing and boiling with the transformation into Nightmare Moon, and Celestia dropped to the floor, wailing as she came to herself and realization struck.
“No! Luna!”
“Quickly, this is good enough!” The shadow spoke. Chrysalis appeared beside the helpless Moon Princess and bound her in changeling slime before Twilight or Celestia could mobilize themselves. Then another shadow, glowing with the eyes of Sombra, covered the Queen and her victim and teleported all three of them away.
“Oh Luna, Luna...” Celestia wept into Twilight’s fur, barely able to speak between gasping breaths and great, choking sobs.