Howdy everyone! I'd like to introduce my own Undertale AU named Mirror Child. In this Universe, the plot of Undertale changes due to a major event that revolves around the known Fallen Child, that is, once again, part of a prophecy. Despite this, the characters' personalities are not changed.

I'll keep with this introduction for now, since I still have to finish Not a "Knife" Feeling. I was too excited to not post this prologue, tbh xD

In the future I might make a comic adaptation of this and even write a masterpost so you're all aware of what the AU is about. Unfortunately, Fanfiction won't let me give you the links of further information about the AU, so you'd better check my deviantART - my account is LotusTheKat and there's a folder named "Mirror Child AU" in my gallery.

As always, I apologize for any mistakes in the writing.

Although there's isn't much to the AU yet, I hope you already like it!

Waterfall wasn't a place that was usually explored. In contrast to the other areas in the Underground, there wasn't exactly a town, instead just a few houses.

Waterfall was pretty lonely, for that matter. Monsters used to explore there more, but ever since New Home was founded, few would actually go back to Waterfall or Snowdin - nowadays, the Capital was gradually becoming crowded and some were already forced to move to another place. Due to this, perhaps Waterfall would gain more inhabitants in the next years.

Though it wasn't much of a problem yet. The Waterfall caves were still that same empty and... creepy place, where Echo Flowers would whisper nonstop into monsters' ears. However, Papyrus couldn't lie that he did find Waterfall a beautiful place, with all the water and shining rocks on the ceiling. It did evoke a strange feeling inside him that he couldn't exactly explain, but it did that to all monsters that would pass by. He wasn't very used to it, though.

It was maybe the first time he was actually exploring the place, thanks to his great and cool friend Undyne, who was captain of the Royal Guard. The two had bonded in a matter of weeks and Papyrus was glad to find someone as proud and exciting as him. Not only that, but she was also willing to train him to become a famous Royal Guardsman! The skeleton couldn't feel any happier to meet someone as amazing as the fish monster. Undyne was glad to be introduced to him as well, which was why she was presenting him her home land so he could already have the map of the Underground in the palm of his hand.

He had to say the journey was fun! He had even met a nice monster named Onionsan, but Undyne was in a hurry so, unfortunately, they hadn't talked very much. Papyrus had also been introduced to a shy monster that sang very quietly, but beautifully. If the two hadn't had such important tasks - such as to initiate their private lesson at Undyne's home, where he still hadn't gone to - he would have sung along. Papyrus was a talented singer himself!

As Undyne and Papyrus walked, the rushing water echoed all over the cave. It was a soothing sound, better than the creepy whispering from the Echo Flowers... Waterfall was such an odd place - they had even found a lonely piano standing in a small room!

"Sometimes I get to play it when I'm not on duty," Undyne said once they saw it.

"OH! YOU PLAY THE PIANO?"

"Yeah, a little," Undyne smirked.

"YOU MUST BE GREAT AT IT! YOU'RE VERY SKILLED!"

"Gee, thanks, Paps." Undyne chuckled, flustered.

"WHY DON'T YOU PLAY IT NOW?"

"Oh, I'd like that, but... I really want to finish showing you around. Besides, I have my own piano at home, I can play it for you once we get there."

"WELL THEN! I'M LOOKING FORWARD TO IT!"

Undyne gave him a wide smile.

"OH! I KNOW!" Papyrus suddenly spoke cheerfully. "WHAT IF WE FORMED A BAND ONE DAY? YOU COULD PLAY THE PIANO AND I COULD SING! AND WE COULD INVITE THAT SHY SINGER TOO!"

"Heh, that's a great idea!"

"I JUST HAVE TO THINK OF A COOL NAME, THOUGH..." Papyrus said, thoughtful.

The two continued their journey while still coming up with a cool name for their future band. Finally, as the two reached the continuation of the description of the War of Humans and Monsters, through ancient glyphs on the slightly wet walls, they changed the topic and talked more about their history instead.

Papyrus read all the glyphs carefully; Undyne had already read them, as she had lived there since she was little. Thus, she would explain him what the glyphs described when Papyrus asked anything he didn't know.

The history behind their kind was awfully saddening, but at least everything would end well - he knew so. The skeleton, as well as many monsters, held tight to their hopes and awaited the day the King would break the Barrier and free them all.

The ones he read in that room described the nature of Monsters' SOULs, even mentioning something named "Boss Monster".

"BOSS MONSTER?" Papyrus read out loud, having never heard of such term before.

"Our King is a Boss Monster," Undyne answered. "His SOUL is a different kind. Physically it's the same white little upsided-down heart, but it lasts longer after death."

"IS THAT SO? THEN HE MUST BE VERY POWERFUL INDEED!"

Undyne nodded in approval. The two then moved forward, as the captain still told him of King Asgore's heritage and species. Papyrus found it all interesting and captivating.

That is... until he found something that caught his attention entirely, especially as he realized how darker the cave became.

Under several waterdrops that fell from the ceiling stood a lonely statue in a dark room, if not for the light that came from above. The sculpture depicted a small figure that had a short hair, the bangs hiding its eyes. Two wings came out of its back as well. Papyrus couldn't exactly tell the species of the creature represented, but it didn't... quite matter.

He found himself staring at it and Undyne's voice became a muffled noise. Only when she drew his attention back to her that she spoke normally to his non-existent ears.

"Are you even listening?" She asked him, sounding a little annoyed.

"O-OH, UM," Papyrus cleared his throat (though he didn't... have one?) nervously. "I JUST FOUND THIS STATUE... INTERESTING!"

Undyne only then understood what he meant once she acknowledged its presence.

"Oh! The statue of the Imprisoned Angel..." She said, seeming that she talked to herself.

"THE... IMPRISONED ANGEL?"

"Yeah! Haven't you heard of it?"

Papyrus shook his head. "I DON'T... THINK SO."

He couldn't help glancing at the statue every now and then. For some... strange reason, it... evoked such strange feelings in him - even stronger than what Waterfall as a whole was making him feel.

"Right, not everyone knows this prophecy..." Undyne murmured.

"PROPHECY?"

"Yeah," Undyne answered. "It's mostly known in Waterfall, though, it's... not that popular. Gerson told me one day and I don't know if I remember the entire story, but I think I can sum it up to you.

"It's about an angel that rose in the depths of the Waterfall caves, after remaining trapped inside a magic tool for thousands of years. One day, a brave monster freed them and they began a very special friendship. Unfortunately, the angel couldn't stay in the Underground any longer, as they... probably had to go to heaven, I think. Then they left the Underground forever, never coming back.

"Nobody has ever seen them, but it is said that the one who met them sculpted this statue to keep their friendship alive. It's the only depiction we have of the angel, since the artist has never revealed themself through other works."

Papyrus didn't actually look at Undyne the entire time. His empty eyesockets gazed at the statue, but he still listened to her.

"WOW..." He said, awestruck.

"Yeah, well... nobody has really heard of this prophecy. Only Gerson and a few monsters know it and we don't tend to share it with anyone else out there." Undyne continued. "Similar to how Snowdin has its own stories, we also have our own legends. It's kind of our thing, but... it would be interesting if everyone else knew of this prophecy, though.

"I think it... kinda helps us have more hope for the future. That maybe... maybe the angel will return and help us get back to the surface one day, because they were very fond of that monster and found love again, after being doomed in a lonely prison of despair for such a long time. It's kinda similar to our own story, right?"

"Y... YEAH..."

Papyrus stared at the sculpture for a very, very long time. He couldn't get himself out of the trance, for some reason. Undyne was silent as well, after telling him the Imprisoned Angel prophecy. It all became very quiet, if not for the waterdrops that hit the structure of the statue and rolled down the angel's face, resembling... teardrops.

However, it didn't stay that way for too long. He eventually returned to reality once he realized Undyne taking him with her to the next room, that contained several umbrellas.

"We better continue our way," She said, then glancing at the umbrellas. "Now, I don't really mind the rain, but I'll get one so I won't wet my clothes. The next room is all soaked."

Papyrus was still out of it, though. His friend became a little worried and shook him slightly.

"Papyrus? Hey pal, you still there?" She called him.

He shook his head and blinked his eyesockets slowly.

"HUH? OH, HELLO!"

"You feeling alright?" She asked.

"Y... YES! OF COURSE!" Papyrus answered immediately, smiling widely. "WHY WOULD YOU ASK SUCH THING?"

"You seem kinda... down."

"I DO? W-WELL! I AM NOT!..." He said, then his voice lost a little of... his excitement. "... WELL, MAYBE IT'S BECAUSE ALL THAT HISTORY WE READ... ABOUT THE WAR AND SUCH... IT MAKES ME FEEL THOUGHTFUL. I DIDN'T REALLY RECORD LEARNING IT SO SPECIFICALLY BACK AT SCHOOL."

"Oh, I see... yeah, it's okay to feel that way. But hey, everything will be just right."

"I KNOW. I JUST... NEVER THOUGHT MUCH OF IT."

Papyrus... kind of felt like he was lying. Well, not that he wasn't captivated by the history of Monsters - he was indeed! However, that... wasn't the main reason why he wasn't feeling really well at the moment. He could tell Undyne wasn't really buying it, either. Or maybe that was just his impression, since the fish monster didn't question anything else regarding the topic.

"It's okay." Undyne assured him with a smile. "Now, come on! Go get an umbrella if you don't want your clothes getting all soppy."

Papyrus merely nodded while she grabbed an umbrella and awaited. He then took one and she already walked forward. The skeleton, however, stopped in the middle of the way. Undyne didn't seem to notice that he no longer followed her. Instead, the future Royal Guardsman looked behind, already seeing the darkness of the previous room that contained the statue.

Papyrus stood there for a few minutes before... heading back there, without thinking twice.

The statue was right there, all wet. The skeleton then thought of doing something to the poor statue.

...

He looked behind the sculpture, finding a gap near the spot that originated the wings. Papyrus carefully put the umbrella there.

Suddenly, he... heard a tune.

Papyrus immediately distanced himself, a little startled. It... appeared that a music box started playing, as he somehow turned it on with the umbrella. The song was... very calming and relaxing, yet... melancholic. The water no longer fell on the statue, as the umbrella protected it.

Papyrus found himself awestruck while listening to the music box, that wouldn't stop playing.

For some reason, he... he felt like he recalled something.

As if he... he knew that song from somewhere.

Suddenly, he started hearing the same tune...

... but it was played through a piano.

He could see... a small figure playing the instrument in front of him.

They played the song without acknowledging his presence at first.

Papyrus approached slowly, and somehow hesitantly, as he didn't quite know if they would be angered at his presence.

The stranger's back was turned at him, revealing a striped, green sweater and brown pants. Their brown hair was slightly messy.

Papyrus listened and approached, raising his arm as a form to draw their attention. At the same time, he didn't want to stop them from playing, as he truly admired the music.

But he knew he had to talk to them.

He... didn't know why.

But his SOUL told him that it had to be done.

He got closer and closer, and closer... and closer-!

"Papyrus!"

Papyrus jumped in fear once he heard his name in such a worried scream. Undyne rushed to him, seeming a little desperate.

"Dude! I-I thought-!" Undyne said, sounding rather pissed yet relieved for finding him.

Finally did the skeleton realize that he left his friend walking on her own. She had probably gone all the way and only then noticed that Papyrus hadn't been there at all. She appeared to have run a lot, judging by the sweatdrops on her forehead.

"O-OH, I'M SORRY!" Papyrus said. "I J-JUST THOUGHT I COULD-"

Undyne looked at the statue, noticing the umbrella that protected the sculpture.

"Huh? W... What did you do?" She asked.

"I JUST... PUT THE UMBRELLA THERE SO THE STATUE WOULDN'T BE SO WET."

"W... Where is this music coming from?" Undyne asked, then looking at the statue. "I-Is it coming from there?"

"I... SUPPOSE...?"

Undyne raised an eyebrow, but immediately shook her head and got back to the point.

"St- Still! You should have warned me!" She yelled at him. "I thought someone kidnapped you or something!"

"I KNOW, I'M SORRY," Papyrus apologized. "I DID THAT WITHOUT THINKING."

"Just-! Just..." Undyne sighed deeply. "... don't do that again, okay?"

Papyrus nodded. "OKAY."

"Good." Undyne breathed heavily, being able to calm herself down. "N-Now, come on, we have training to do!"

She grabbed him by his hand before he could even say something, probably to not let him out of her sight again. She handed him her umbrella, no longer caring if her clothes were getting wet. She didn't mention anything about the music box, though, so he didn't bring this up again.

Papyrus was rather quiet the entire time, while the "rain" fell. His boots step on many pools that filled the ground. He could even see his own reflection in them. He couldn't quite admire it all, though, as Undyne kept an eye on him the entire time and rushed them to move forward.

Meanwhile, the music played from far, far away... Papyrus could still hear that same piano somewhere as well, as if...

... as if the stranger were still there playing.

The song that plays from the music box would be like the theme from the PS4 version of Undertale (you can look it up on Youtube since, again, Fanfiction won't allow me to put the link here)

Now, I'm not quite sure when I'll get to post the next chapters, but, hopefully, I'll get to work better on this once I finish Not a "Knife" Feeling.