Nightmare wandered through the Batcave, letting her mind wander over everything that had happened recently. Between the various villains Rarity had reformed, Terry's training as Batman, and the Justice League's focus on dealing with the Joker chips, it was plain to Nightmare that Rarity had accomplished her goal: Gotham didn't need Batmare anymore.
While this would have been easy enough to accept and opened the door for Nightmare to bring these events she'd set in motion to a close, there was a problem she had not anticipated. Gotham still needed Rarity Wayne.
Rarity had been spending less and less time under the mask, having handed those duties off to Terry for the most part. Instead, she focused on using her position as head of Wayne Enterprises to clean up Gotham in other ways. While Batman focused on taking down those criminal elements that still sought their criminal activities - at this point mostly the Jokerz, and the occasional criminal family - Rarity focused on addressing those issues in Gotham that created the criminal element, focusing her attention on Lower Gotham, where the slums were, and finding the rare gems of talent amongst the sickly and downtrodden as she made the areas healthy to live in with company resources.
All in all, Nightmare was bored senseless. She had greatly enjoyed the combat against supervillains, and the chaos of the unknown it brought with. She felt what she should really be doing at this point was working the magic to take them both back to Equestria - now that relativistic time had allowed Rarity to catch up to herself - and leave Gotham to its own devices. But she wasn't.
Instead, here she was down in the Batcave working more powerful magics, though still ones within her power, to reopen the original portal that brought them here. If all she wanted to do was open a one way gate to take them home, she'd be able to do so easily. But for some reason, she wasn't.
She knew why, of course. Despite everything, Rarity's enthusiasm for saving the city had raised her curiosity. As much as she genuinely wanted to get back to where some excitement was bound to happen, she desperately wanted to know if Rarity could actually pull it off. It made her regret making that wish all those relative years ago - when the power of Harmony kept her caged deep inside an unaware dressmaker - that brought them here.
Never trust a draconequess, she grumbled silently as she approached Vault E-G, the one Rarity had arrived in. The magic that had made the portal on this end had been drawn from the magical artifacts stored there, but it hadn't been absorbed by the portal or by Rarity. It had become embedded in the very walls, floor, and door of the vault, waiting to be reactivated. Being able to kibbitz isn't the freedom I wished for in my book.
Approaching the vault door, she began drawing runes upon it with her horn, activating the magic contained within. Once completed, the circle she was drawing would begin the slow, gradual process of stabilizing the wormhole between here and the other end, in Equestria. When it was complete, what once was a portal would be a bridge.
By the time she comes down here again, it'll be complete, Nightmare thought to herself. And then it will be time to go home. She allowed herself a pleased smirk as she finished drawing the circle and watched the door begin to warp as space-time rippled from the focused magic that was gradually digging a hole through its fabric. I wonder how the Princesses will react to Dark Night. And I wonder how Fa- She cut her thoughts off. How Bruce will handle Equestria. He'll be able to pass through too, after all. It might just give the old fool a heart attack!
She tried to be amused at that last thought, but found instead that it was disturbing. She cursed the parasitic nature she had taken on after being purged from Luna, from whom she had been born. She existed only to find a host, and then her own goals and desires became shaped by those of her host. And Rarity was making her soft.
Maybe if I reveal myself at an inopportune time, they'll find a way to purge me from her, Nightmare grumbled. Then I can come back here and convince Bruce to be my new host or something. Or maybe use magic to turn a potato into a person. Joker would be a nice vessel for a while. Hmm...Nightmare Joker, no, not a good ring...Dark Humor...oh, I like that. Dark Humor, the Comedy Queen of Crime...
Letting loose a loud yawn, Nightmare returned to Rarity's bedroom, letting herself slip into her own slumber and letting the body revert to Rarity's slumbering form.
Several days later, after taking her turn patrolling the streets of Gotham, Rarity noticed something odd in the Batcave. "Father," she said in concern through the comms, "why is there a second staircase in the Batcave?"
"Beg pardon?" Bruce asked, surprised.
"In amongst the vaults, there's a stairway going up where there used to be a door," Rarity explained. "The steps are also much smaller than the ones leading up into Wayne Manor."
"Smaller in what way?" Bruce asked.
Rarity approached the steps. "Well..." She examined them for a time. "They almost seem sized to me, as strange as that sounds."
Bruce was silent for a time. "Rarity, go up those stairs, but proceed with caution."
"Are you certain, Father?"
"Yes. Keep up a running commentary for as long as communications hold."
Rarity paused with one hoof hovering over the bottom step. Her mind ran fast as she interpreted her father's words. "You think this leads back to the world I come from," she intuited. "And you aren't sure if I'll be able to come back?"
Bruce was silent for a time. "There is a distinct possibility of that, yes."
Rarity put her hoof back down on the floor. "Then why should I go?" she asked quietly.
"Because if that path to your world has opened, your world likely needs you," Bruce replied. "And I know you won't shy away from that."
Rarity winced. "But...but what about Gotham? And Earth?"
"If we continue to need you, I'm certain you'll find your way back."
Rarity sighed. "Alright, Father. ...wish me luck."
"Good luck."
With those words exchanged, Rarity began climbing up the stairs.
When she reached the top of the stairs, Rarity found a doorway, again sized to someone her size rather than a human. "I've found a doorway, Father," she said over the comms. "It's sized to me as well. ...I'm opening the door."
"Be careful," Bruce cautioned.
Rarity pushed the door open. "There's another stairway leading upward," she explained, "though this one looks more normal. There also appears to be...some sort of basement." The door shut behind her as she stepped through, and she saw the glow of runes. "Father?"
"I can still hear you," Bruce replied, though there was a strange quality to his voice over the comms, as though it echoed from a great distance. "However, the electronic communications have cut out. We're on the magical backup Rachel and Victor designed for you."
Rarity breathed a sigh of relief. Stepping forward, she turned the doorknob of the rune inscribed door, and found she was able to open it easily, revealing the staircase back down. She let the door shut again. "I'm going to investigate further," Rarity explained. "However, this may be a private residence of some sort, so I won't be speaking further."
A good idea, Nightmare mentioned. It's nighttime here, and we wouldn't want to wake Sweetie Belle.
Rarity flinched at Nightmare's statement, but narrowed her eyes. You know what's going on-
Go upstairs. It'll become clear soon enough.
Frowning, Rarity walked up the stairs. Stepping out of the basement, she found herself in someplace very familiar. "This..."
Yes, Nightmare replied. This is the place from your mindscape. This is your original home and place of business...Carousel Boutique.
"Where everything is chic, unique, and magnifique," Rarity found herself saying before gasping and covering her mouth. How do I know that?
Nightmare chuckled. It seems your memories weren't gone, just deeply suppressed. The right triggers will bring them back...and I'll be able to manage their awakening as they come so you aren't overwhelmed.
And what's in it for you to do that? Rarity demanded silently.
It's no fun if my host is reduced to a vegetable, Nightmare pointed out.
"R-Rarity?" a quiet voice whimpered. "I-is that you?"
Turning, Rarity saw a small white unicorn filly with a pink and purple curly mane and green eyes looking up at her nervously. As memory bubbled to the surface - gently cushioned by Nightmare - Rarity smiled, recognizing the filly. "Yes, Sweetie Belle, it's me. Did I wake you?"
Sweetie shook her head. "I heard Discord laughing, and some strange noises...and then you were talking to someone. Umm...why are you dressed like that, sis? It's nowhere close to Nightmare Night." Her eyes widened as she saw something she didn't expect. "Rarity...you have bat wings? When did that happen?"
Rarity chuckled as she pulled her mask off. "It's...a bit of a long story, Sweetie Belle." She smiled. "How about I tell you some of it, to help you sleep?"
Sweetie smiled widely, letting Rarity guide her back up to her bedroom. "It's been forever since you told me a bedtime story!"
Rarity chuckled. "Well, I think you'll enjoy this one. It begins with a bat who could no longer raise his wings..."