Hyperdimension Neptunia belongs to Compile Heart, Idea Factory, Sega and whomever else worked on it.

I blame a random comment I got on Discord for this.

"Hey! Histy!" Neptune smiled as she walked into the Basilicom. Unlike how she was normally, her hoodie had small tears and some dirt on it, as did her face and hair. "I was out having a picnic today when the ground broke open and I fell into the hole."

"Oh dear." Histoire frowned in concern. "Was anyone else there?"

"Hmm... Nope." Neptune shook her head. "I already called Iffy and told her what happened, so she could call the right people to seal the hole up. But I thought, since I was down there, I might as well look around, in case a monster was around."

Histoire nodded and fought back the urge to smile as she listened to Neptune. "I see, and were there any?"

"Hmm, no, that's the weird thing, it was just a small cave but it had this really old book in the center that was glowing and everything." Neptune reached into her pocket and pulled out a slightly beat up and thick brown book with a cross on the cover. "It's weird, cuz it feels like this book has something inside of it."

"Indeed," Histoire nodded as she looked at it. "It is interesting." She gave Neptune an accusatory look. "You didn't try anything stupid, did you?"

"Eh..?" Neptune had a flat look on her face. "By stupid, do you mean when I smacked it with a Cyprus stick? Or when I sang to it? Or when I tried tickling it like I did that one time to you when you were sealed in your book and had the hiccups."

"Don't remind me about that." Histoire said, an annoyed look on her face. "And it seems that you didn't do too much." Though singing wasn't too bad of an option, Neptune wasn't always on key, but she did have a pretty good singing voice.

"Oh! I know!" Neptune's eyes started to sparkle. "Let's use that new scanner and see if there's something inside!"

"That seems like a good idea." Histoire nodded and the two made their way to a secure room to put the book in before Neptune ran off to get the scan device.

The only reason they were in a secure room was because the last time they tried scanning something that wasn't in the secure room, the entire Basilicom ended up covered in foam...

...Which ended up covering every building in a three block radius from the Basilicom. The complaints of the citizens had been non-stop for weeks while they cleaned it up.

"At least everything was super clean afterwords." Histoire muttered to herself. "Now that I think about it, we still haven't had to clean any of the buildings that were affected by that, have we?" It almost made her want to know what that foam had been made out of and make some to sell to Lastation.

"I'm back, Histy!" Neptune smiled as she walked in with a device with Nepgear following right behind her. "And Nep Jr. offered to help since she wanted to know what was going on."

Nepgear smiled as she helped set up the scanner. "So this is this book, huh?" She asked while looking at the brown book and Histoire nodded. "Do you think it's another tome like you or Croire?"

"Perhaps, but I hope if this one is that it's not like Croire." Histoire muttered softly and the Nep sisters nodded, knowing that another Croire-like being would only cause problems.

"Let's see... Oh!" Nepgear's eyes widened and her eyes started to sparkle. "This is... It's more like a computer storage device than a tome like Histoire. Oooh! There's actually magic in this, but it's magic that uses pretty advanced math." Nepgear tilted her head in confusion. "Huh, this would actually make learning magic a lot simpler for some people, though I'm sure that Rom and Ram would hate it."

"Yeah, they don't like school much, huh?" Neptune sweat-dropped at the idea of Rom or Ram trying to use math to do magic. "Anything else in there, Nep Jr.?"

"Hmm?" Nepgear tilted her head in confusion. "It looks like there's a really complex program in here and... Huh? What's with this corrupted data?" Pulling out her N Gear, she plugged it into the scanner device and tapped the screen a few times. "Wow! It's like someone was trying to make modifications to a computer program but had absolutely zero idea how to even turn on a computer." She frowned softly while working on it. "Ooooh! I'm not good with software like this."

"Can I look, Nep Jr.?" Neptune asked and looked at the N Gear. "Yikes! It's like the person was dumber than me!" She gently took the N Gear out of Nepgear's hands and touched a few things on the screen. "How about we isolate this and then we do this and... Ah, there we go! I deleted all the corrupted data." She blinked at the shocked look that Nepgear gave her. "What? You do realize that I made our nation's console, right?"

Nepgear sweat-dropped heavily. "But you don't, I mean..."

Neptune smiled at her. "It's okay, kid-sis-o-mine, I don't know too much, but I can do some mean programming... Once in awhile."

"And only when your motivation is high enough." Histoire muttered, glad that Neptune could do this much. "So what..." She stopped as the book started to glow brightly. "Neptune, what did you do?!"

"NEPU?!" Neptune cried out as the glow got brighter and brighter before exploding outwards and knocking Histoire, Neptune and Nepgear far away from the book. "Wowza! That was worse than that time that your Nepgeardam self-destructed!"

Nepgear couldn't answer as the explosion had kicked up a lot of dust and obscured everyone's vision. "What," she coughed a few times, "happened?"

"That's what I would like to know." A new, female, voice spoke up. When the dust cleared away, Neptune, Nepgear and Histoire were surprised to see a woman that was about as tall as Vert wearing a black mini-dress and black gloves. Her hair was silver and white while her eyes were amber-red and, much like Vert, she had a very buxom chest, though it wasn't quite as big as the Leanbox CPU's boobs were. "...Where am I? The last thing I remember was..." She gripped her head and grimaced. "...Not much, actually."

"Wow, Histy!" Neptune's eyes sparkled as she looked at the newcomer. "This tome's much bigger than you are."

"Tome?" The woman blinked a few times. "Yes, that name seems familiar... Tome of the Night Sky... Reinforce."

"Your name is Reinforce?" Histoire asked, getting a nod from the woman.

"Yes, I..." She frowned in confusion. "I don't know why, but I feel like I'm missing so many memories."

"ooooh!" Neptune nodded in sympathy. "Amnesia, huh? I know that feeling very well. Well, Miss Reinforce, why don't you stay here at the Basilicom until you get your memories back?"

Reinforce blinked at Neptune's smiling face and blinked a few more times as she thought she saw a young girl wearing a white dress who had red-brown hair done up in small pigtails standing where Neptune was at. "...I think I'll take you up on that offer."

Reinforce is from Magical Girl Lyrical Nanoha A's and is known as the Book of Darkness, originally known as the Tome of the Night Sky, but she was corrupted and turned into a weapon of mass destruction. Good thing Nep fixed that, even if she caused poor Reinforce to suffer amnesia.