Everypony was silent, staring at Zecora. Applejack and Dienestus peering over from beside the door, Rarity actually having looked up from examining the curtains beside the large picture window, and the princesses actually approached Zecora. Princess Celestia allowed a look of confusion to overtake the worry in her face.
"I don't understand, Zecora. What do you mean Luna is 'held captive by Nightmare Moon?' Twilight and her friends unleashed the Elements of Harmony upon Nightmare Moon and restored Luna to her old self. How could Nightmare Moon still exist?"
"Nightmare Moon and your sister are not one in the same," Zecora started. "However, they existed together before she gave herself a name. The mind is a curious thing, your Excellence. It very seldom makes any sense. When two personalities co-exist, within the mind there is always conflict. Or a battle, if you prefer, and in Luna's case, the victor was almost always her."
"Nightmare Moon was a corrupted form of Luna, not a whole other entity inside of her mind," Celestia argued. "... wasn't she?"
"Some ponies have more than one personality," Twilight translated. "A friend of Pinkie's back in Ponyville has four. What Zecora's saying is that Luna has always had this separate self inside of her mind, and for a long time was able to keep her controlled, except for when her jealousy got the better of her."
"... when she was young, Luna sometimes acted uncharacteristically rude, and mean, even for her. She would be aggressive, and moody, and she would even break things. And she would swear that she didn't remember the things she did or said when she was in that kind of state. I always thought she was just trying to get herself out of trouble, or that maybe it was her illness making her behave that way..."
"Ponies with multiple personalities can often lose time when the alternates have taken over. It's entirely possible that Luna didn't remember what she'd done," Twilight explained. "I don't know how much of it was Luna being jealous, and how much of it was Nightmare Moon's influence, but I've seen the night you banished Luna, and she didn't seem herself at all. I think the personality that called itself Nightmare Moon just got strong enough to break free."
"Alright. But can we save her? Can we bring her back?"
"Since Nightmare Moon got out before, Luna knew of what she was capable. And so it would seem that she has tried to contain her, to keep her relationship with you, her sister, stable. As a result, she is not present in our reality, but in one of her own. By travelling through her past and searching for answers, we might be able to bring her home."
"When you and Luna were being held captive by the vines killing the Tree of Harmony, Zecora made a Memory Potion that let me view the past. Maybe we can try that again, and I can try and see how Nightmare Moon gained power over Luna and escaped her mind to manifest as a new consciousness."
Celestia looked worried, but eventually nodded. "Alright. Alright, whatever it takes to save my sister."
"It takes one hour to mix the brew. In the meantime, you must choose who will drink the potion; Twilight, or you."
"I will," Twilight piped up. "I've done it before, and I can observe from a neutral standpoint because I wasn't there."
"Twilight, are you sure?"
"Yes. I want to help Luna in any way I can."
"Alright. Zecora, come with me to the castle apothecary, we'll find you the ingredients you need. Twilight Sparkle, you and your friends stand watch with Luna's guard in her bedroom. We will join you once the potion is brewed."
"Yes, Princess Celestia." Twilight glanced at her friends, who all stood up and trotted out behind her.
-
After the potion was brewed, the older mares appeared in the room, presenting the completed formula to the youngest princess. The white potion levitating in front of her, Twilight gave the same nervous gulp she had the very first time. She didn't know where the potion would take her, and she had no idea what she would see. She took in a breath, then, with a nod to her mentor, took a large sip of it.