A/N: Hi there, fellow Undertale fans! As a note to start this off, you don't need to be familiar with Phantom of the Opera to understand this, it can stand on its own. There will be some comparable elements, both character and music-wise, but at the heart of the story is the Undertale aspects.

Basically, I've loved Phantom of the Opera since the 2004 movie came out, and I've gotten back into my obsession with it, just as with Undertale. Then a few days ago, as I was dwelling on my ships for Undertale, and thinking how I mainly ship Chasriel but I love Frasriel too (not sure if that's the right term for FriskxAsriel, haha), I thought, "Hey, Phantom of the Opera is a love-triangle thing I really love, and Chara would make a perfect Phantom! Why don't I make it about her, Asriel, and Frisk?" So the thought of doing that filled me with determination. :)

Basically, in this AU, humans and monsters are both integrated into society, the opera house/theatre is in the Underground, and the backstories of some of the characters are thus a bit different. Well this be a direct Phantom-of-the-Opera-with-Undertale-characters thing? Nope, and there will be some storyline twists and background changes, as well as a few surprise characters. ;) But as far as core character similarities go, basically Chara is like Erik/the Phantom, Asriel is like Christine, and Frisk is like Roaul—for the record, I definitely ship ChristinexErik PotO-wise, but I love Frisk too.

Finally, regarding music—I've been watching the Undertale the Musical videos recently, and Phantom of the Opera is a musical. This won't really be a song fic, though I may insert one or two songs in there, and if I do, it will likely be to the tune of an Undertale song. Again though, there likely won't be many, if any at all. Heck, I may include Mettaton's. XD

With all that said, on with the fic! :)

Disclaimer: I don't own Undertale, or Phantom of the Opera.

The few remaining occupants of the Underground Opera House knew that within 12 hours, the atmosphere would be transformed from a silent stage to a building that blazed with life, both human and monster.

To the casual observer, a visitor would only think there was one remaining monster in the Opera House, an orange cat-like being called Burgerpants in the backstage rafters. He tugged on the rope to hoist the wooden plank upward, adjusting his supplies to make sure that everything was nicely polished and ready for the next day.

"Can't have any of the actors getting splinters tomorrow," Burgerpants muttered, turning a careful eye to the floorboards as he walked onto the upper layer where curtains and backdrops could be dropped down for each show as needed. He let out a laugh, somewhat bitter but with a twinge of anxiety. "I should be one of them," he exclaimed, raising his voice to fill the silence and keeping his ears trained for any suspicious sounds. "But no, they said 'you need to work on your act, you're bette suited for backstage work'." His voice took on a mocking tone before he shook his head in defeat. "Maybe the new managers will be nicer to me."

"I would hope so if I were you. Luckily I'm not."

The voice startled Burgerpants so much that he dropped the hammer just inches from his foot, his cigarette also dropping to the floor. Leaping up a little, his fur standing up on end both from the shock of nearly having a hammer driven into his foot and the voice he heard. Hands shaking, he gripped the hammer as a weapon, eyes and ears trained as he fought against the urge to swing it wildly around.

Not that an action like that would do anything against a ghost.

Burgerpants flinched a little, looking up in the rafters as the smallest of scraping noises reached his ears, though no other voice spoke. Part of his mind thought it could just be a rat, but he knew better, Momentarily pocketing his cigarette, he trained his attention up to the rafters. He knew he had heard something—someone—and he knew exactly who it was. Even if some refused to believe she even existed.

With the opera house under new management, maybe it would be different. Perhaps he could get that Dr. Alphys to set up cameras...

But the monster shook the idea out of his head the moment the thought came to him. Encroaching on the staff's privacy would be a bad idea,and who was to say she would even show up on film? The years he had worked here as the janitor didn't give him many clues, and he already had more than enough situations where that thing would reach into his mind and invade his dreams.

At least members of the opera's staff knew, a good number of them believing. The patrons, though...

But having others who believed the rumors about the Phantom wouldn't help him here. Right now it was just him and the ghost. And not a friendly ghost like Napstablook. Blooky and that thing were of a different incorporeal nature entirely, or so Burgerpants believed. Blooky was a friendly ectoplasmic manifestation, and while it was true he had self-esteem as low as the bottom of the ocean, he wouldn't hurt a fly. The Phantom, on the other hand... Burgerpants got the feeling that she could kill him as soon as look at him, even though those times when he did sense her nearby, it was nothing more than a voice of a few short words and a momentary flicker of red eyes glinting in the dark like their owner was about to leap out and devour his SOUL any moment.

Swallowing, he tried to ignore his racing thoughts and mounting fear, only to have the voice speak again.

"So," the voice said thoughtfully, so quiet it could almost be mistake for a conjuring of the imagination. "Opening again tomorrow." A light giggle echoed. "About time."

Burgerpants stood up straight, his fingers clenching tight around the hammer. If this Phantom was going to try to bully him, he wasn't about to let it—at least that was what he tried to tell himself. If only one of the others was here, then he wouldn't seem so crazy.

"I know you're there," he called, trying to sound braver than he felt. But though his voice was stronger than he had expected, his shaking legs gave away his fear. "I know you're there you... you phantom..." To call her a human or monster would be an insult to both races. If that thing had been one or the other, long ago, she wasn't any more. A demon would probably be the best word to describe her, but the thought of doing so only further terrified him. "Come out, Phantom."

But nobody came.

After a minute of silence, hear and seeing no indication that the being was still present, Burgerpants turned away from the ceiling and nervously began humming to himself as he swept the floor. Even so, he couldn't shake the feeling that eyes were watching him, melted in the shadows.

He wasn't wrong.

In fact, such a tactic was practically the owner's area of expertise. That, and scaring the living daylights out of people on occasion when she was bored. Whether unintentionally or not.

The figure scrunched her eyes shut for a moment, unseen as always. Her gaze briefly flickered downward to where the empty stage could be seen, looking as if it had been abandoned for a few years rather than hours. Quiet and boring, but good for giving a good scare to anyone who happened to be out after hours.

Burgerpants was always good for a laugh. Everyone thought the 19-year-old was a bit insane anyway, it wasn't like anyone unfamilar with the opera house would take his claims of a "phantom" as being worth serious investigation. But then, no one had really bothered her for years. Why start now?

She hoped that, perhaps, the grand re-opening would bring a change around this place. Scaring the jumpy janitor could only go so far for entertainment, and the new furnishings and props would certainly bring this area to life again. Maybe even some more fresh talent. Supposedly a few members of the choir were set to try for solos.

But she could only think of one she hoped to see there. Even if he'd never really see her—but hearing his voice again would at least be a blessing in this maddening silence..

"I can come back."

"You won't be able to until it re-opens again. It's only for a few months." The words hurt to say, but she wasn't about to let any weakness show in her voice. She had many traits, but weakness was certainly not one of them. "Just remember everything I've taught you."

"Maybe we could meet outside the—"

"No!"

She had immediately regretted the forcefulness of her response when she saw the young goat-monster leap back slightly at her tone. She quicly tried to recover herself and try again—given that he had only heard her voice, she didn't want their bond to be tainted by a stray outburst. "I have to stay here. I'll meet you once the place re-opens."

"But—"

"Please. For me."

There was a pause. "Alright."

"Promise?"

"... I promise."

The reply was reluctant, but for her, it was enough for the time being.

Bringing her thoughts back to the present moment, the spectre brushed her brown hair out of her face, the strands of hair lightly brushing over the mask. The red light from her eyes gave almost ethereal glow to the wooden beam that she lay against, but by the time she heard that pesky janitor turn to look up, she had gone.

Moving silently through the rafters and crevices of the building, descending from the stage and venturing deeper than where anyone else dared go, a smile crossed her face.

Tomorrow the Underground Opera would finally be open again.

A/N: Yup, Burgerpants is the basically like Joseph Buquet in this AU, he was just the first one that popped into my head for that—I'm actually surprised myself at how much of a role BP got in this first chapter, it wasn't my original intention but I just kind of ran with it. XD And don't worry, Asriel, Frisk, and the rest will appear in the next chapter. And for those who are Phantom of the Opera fans, see if you can guess who will be like which Phantom characters, because there will be at least a few parallels, though I have plenty of twist ideas for this story. ;) Anyway, see you next chapter and reviews are appreciated!