Twilight wandered the hallways aimlessly, beginning to wonder why she had woken up in the first place. She turned a corner, and in her drowsiness nearly ran into somepony before realizing they were even there. She quickly looked up into the eyes of Princess Luna. She sighed.
"Good evening, princess," she spoke softly, the events of the last couple days cowing her usual exuberance.
Even with the magic seal, Dusk pushed against the edges of her consciousness, waiting for an opportunity to take over. The effort it took to keep it out took vast amounts of her energy. However, without magic to practice with, there was not much else she needed to do, anyway. Eat, read, be with Trixie, and sleep was 90% of her day, the other 10% being talking with Luna about how she might expel the Nightmare for good. So far, not much had come up.
"Twilight, I keep telling you, I'm fine with you calling me Luna. Anyways, I was actually coming to find you. I think I may have found something that, while rather extreme, might help you with getting rid of Dusk." Twilight immediately threw off her sleepiness, staring wide-eyed at Luna.
"R-really? That...That's fantastic!" She seemed a bit more energetic now. Princess Luna nodded, then turned back the way she had come, obviously meaning for Twilight to follow. She did, of course, curious and eager to be rid of Dusk once and for all. They wandered the halls, seemingly at random for a while, before stopping before the door to the artifact room, where anything either dangerous or worth studying was stored. Twilight became nervous, and tapped Luna's shoulder.
"Umm, Luna. If you don't mind me asking, what have you found, exactly?" Luna said nothing, and Twilight grew even more nervous.
Without any further pause, she stepped forward, sliding her horn into the door to unlock it. It shimmered, then disintegrated, allowing access. Luna stepped inside without any hesitation, motioning for Twilight to do the same. Twilight, still nervous as hell, stepped forward slowly. There wasn't much light in here, so Luna lit her horn with magic, casting a deep blue illumination on the entire room. Satisfied, she stepped towards one particular item. It was simply labeled, "The Harmony Key."
"I've never heard of this 'Harmony Key' before. What's it going to do to the Nightmare?" Twilight's ever-present inquisitiveness took hold. However, again, Luna was silent, and simply lifted the item from it's case. She levitated it over to Twilight, who took it in her hooves as it reached her. Immediately, pain seared through her, flowing from her hooves to her shoulders, her chest, then up to her brain. She screamed, and in a flash of black-purple light, disappeared to Celestia-knows-where. Of course, Celestia herself knew not where she went. Luna had an idea, however. She grasped the floating threads of the teleportation glyph that had been weaved over Twilight when the spell took hold, and used them to follow, weaving it back together with her magic.
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Trixie woke up and looked around, slowly and groggily, for Twilight. Not seeing her, she assumed that she had just left her to sleep and went for breakfast. She walked out of their room, walking facefirst into Celestia for the second time since arriving there. She scrambled into a bow when she realized her mistake.
"I'm sorry, princess, I was just looking for Twilight, she isn't in here." Celestia, however, levitated Trixie off the ground to look her in the eye. She looked furious, but also worried.
"I had just come to see if you'd know anything about Twilight and Luna meeting last night. A guard saw them go into the artifact room, but nopony came out. Eventually, he came and got me." She dropped the scared filly suddenly, backing up a couple steps. She shook her head.
"I'm sorry, Trixie, I am just worried for them both. I went in to look for them, but neither of them were inside. The Harmony Key is also missing," she said, the last almost an after-thought. Trixie ignored that, focusing on the fact that Twilight and Luna were both missing.
"You don't think that Dusk broke out of the seal already, do you? It could've taken Luna captive." It seemed a quite logical explanation, however, Celestia simply shook her head.
"I can feel the seal, so if it broke, I wouldn't be...able to...track it." She thought for a moment. If she could feel the seal, she could teleport to the origin of the signal, A.K.A. Twilight. There was no guarantee Luna would be there with her, but she'd take one over zero any day. She pulled Trixie close, weaving a teleportation glyph of her own. With no clear destination in mind, she had to reinforce it with a seal to make sure they didn't teleport into a wall or something equally dangerous. However, in a few moments, the spell was ready, and she immediately poured in the magic to start it, and they were off in a flash of white light.
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Twilight hit the ground, hard. With a grunt, she rolled onto her hooves and stood up, then blinked as a flash of dark blue signalled Luna's arrival. The Princess, however, had landed on her hooves. She picked up the Harmony Key with her magic, tucking it into a pouch on her hip. With that, she turned and walked in the only direction available. Twilight groaned, following without much of a choice in the matter. She stumbled multiple times, still getting used to the flares of pain in her foreleg, as it still hadn't fully healed. Unicorns, without magic, had very slow physical processes. Twilight finally, after about a half an hour, collapsed, almost brought to tears by the pain she was bringing by abusing her leg in this manner. Luna sighed, pulling Twilight onto her back. Then, she galloped, going much faster without being slowed down by the injured filly. She wrapped her forehooves around Luna's neck for balance, barely able to hold on.
Finally, after an interminable time spent going down one hallway after another, they arrived at Luna's destination. Twilight slid off her back, trotting forward into the room they had reached. In the center, trapped in a beam of light, was a black pendant with a small ruby in the middle. Twilight looked over at Luna, who nodded, smiling for encouragement. Twilight, shrugged, turning back to the artifact before her. It seemed to radiate power beyond comprehension, which made Twilight wary. Still, Luna thought it could help, so she took it in her hooves, then clasped it around her neck. Then, she sat there, waiting for something to happen.
After about ten minutes, Twilight stood up. At the same moment, a flash of white light blinded both her and Luna, causing Twilight to stumble back into a wall. Slowly, her vision returned, revealing Celestia and Trixie on the far side of the room. They looked around, trying to ascertain where they had been teleported to. The amulet flashed at her neck, and Twilight's vision flickered with red for an instant, but went back to normal. She thought nothing of it for the moment, and turned to Luna.
"Nothing happened. How is this going to help me, anyway?" She was slightly suspicious now, but Luna merely chuckled. She turned to face Twilight, and the look on her face confused Twilight.
"Help you? Dear, sweet Twilight, do be patient. All will be clear, and quite soon." With that, Luna slowly trotted over to her, while she tried to back further into the corner. She didn't know why, but she really didn't like the look on her face. She was going to scream. She opened her mouth to do just that.
Before she could, though, a flash of light blue put Trixie in front of her, facing Luna. "Get away from her, can't you see she's scared," she screamed in Luna's face. Luna was taken aback from the outburst, and looked at Twilight.
"Are you really scared, Twilight Sparkle? Are you truly?" Luna spoke as if knowing Twilight's true emotions, though that should be impossible. However, she knew what was going on, more than anypony else in the room. Twilight stood up suddenly, a red glare masking the purple they should've seen as the seal shattered, filling her with pent-up magic and, unfortunately, Nightmare Dusk. The red was replaced with black, then they flickered between the two as Dusk fought the amulet for dominance of Twilight's body. However, Twilight herself couldn't remain in control, either, and had already been locked into a small corner of her own mind.
~
Dusk dug it's hooves into the dirt, having finally fought the Amulet down successfully. It's horn flared with magic, tossing Trixie aside easily, pinning her to the wall. The black-purple glow of magic was unmistakable, and Celestia was taken aback, finally feeling her connection to Twilight's body shatter. She ducked as a blast of blacklight hit the rock behind her, melting it instantly. Damn it! Dusk thought, blasting again and again, trying to get one good hit on the white alicorn. If it could hit her once, she would never trouble it again. Let me help you, the Amulet whispered. They have no chance if we unite our power. Together, we could easily win this battle. Dusk sighed. This better not be a trick, it said, stepping aside a little so the Amulet could help.
That done, they turned back to Celestia, blasting blacklight at a renewed pace, leaving Celestia barely able to dodge most of them. She erected a small shield as one hit her square in the side, melting away at the barrier. She pumped more magic into it, sealing it back up, before springing out of the way of three more. She was tiring quickly, much quicker than she had originally anticipated.
"Luna, dear sister...Help me." Celestia called out to her, and she readily agreed, beginning to try and tap Celestia's connection to Twilight's mind. Maybe she could do something from there. She slipped in the back door of her mind.
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Almost immediately, she came across Twilight, battered and weak, in her mental cage. She hastily freed the unicorn's mind, giving it some of her strength, before they went to confront the two invaders. This had been Luna's plan all along. If they could defeat the Nightmare here, then Twilight could take the amulet off, freeing herself from it's influence. Luna blasted at the Nightmare with her magic, catching it square in the back. The Amulet turned to fight them, leaving the Nightmare in control of Twilight's body, and immediately caught twin bolts of magenta and blue as Twilight, having her magic returned to her, and Luna cast at the same time. The Amulet shrieked with fury, causing itself, the Nightmare, and Twilight herself to cringe in agony, rendering the body uncontrolled for the moment.
Twilight, however, was the first to recover. She dropped Trixie off the wall, then reached behind her and undid the Amulet. It shrieked again as it flashed out of existence in her mind. She threw it away before returning to the battle with the Nightmare. However, in those few seconds, the Nightmare's power had nearly thrown Luna out of her mind. No, she screamed, causing herself and the Nightmare to cringe yet again. Luna, however, seized the opportunity, flinging herself back into the fight. She blasted the Nightmare twice more in the back, causing it to catch fire. Twilight, however, was nearly out of strength once more. She was gasping for breath, weakly attempting to blast the Nightmare herself, but it did no good. Luna quickly told her to leave the fight and instead focus on maintaining control of herself. She nodded, flickering out of existence in her own mind as she returned to reality.
~
Luna returned her attention to the weakened Nightmare. It would do no good to seal it up again, it had to be expunged completely from Twilight's mind, or all this would be for naught, and it would simply return again. She channeled a very complex spell, weaving the glyph above her head as she advanced. She touched the Nightmare with her hoof, ignoring the searing pain from it being on fire, and completed the spell. It threw her and the Nightmare out of her mind, and without a host, this Nightmare disintegrated. Luna returned to herself.
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Twilight and Luna staggered to their hooves, and Celestia, weak from exertion, was about to fall down. They looked to Trixie. She sighed, already weaving the glyph to take them home. It would take most of her magic, but she guessed it could be done. It took a minute for her to correctly calculate how much magic it would take. Then, she poured in the magic, and in four flashes of light blue light, they appeared in Twilight's room, which was the only target Trixie could make reliably in the castle. They sighed with relief, Twilight crawling into the bed, while Celestia and Luna walked out without a word. Trixie walked over to the bed, prodding Twilight with a hoof.
"So, is it...over," she asked hesitantly. Twilight nodded and pulled Trixie to her tightly. She sighed, nuzzling Trixie's mane as she went to sleep. Trixie giggled, stroking Twilight's mane as she fell asleep as well.