Hey guys! I decided to try Something a little different. I discovered the game "Undertale" a few weeks ago and I became such a fan of it! 3 So, I thought what if its universe was implemented into "Once Upon A Time"? It doesn't have to be just about fairytales, after all. That's why I decided to write a crossover. ^_^ I feel very inspired for this story and I really hope that you'll like it. I'll try to update regularily.

To clarify, this is an AU to the canon of OUAT season 6. Basically, the season 5 finale and the whole season 6 never happened. But a few elements from it still do exist. Like, Emma and Hook are engaged, and Regina is mad with Zelena.

Disclaimer: I own nothing. This is a fan story written just for the pleasure of writing and having fun with my two favourite universes. "Once Upon A Time" belongs to Adam Horowitz and Edward Kitsis, and "Undertale" belongs to Toby Fox.

Underground

In king Asgore's castle, a device was being installed next to the "barrier". The king was constantly asking Dr Alphys if everything was going as planned, to which the latter would always answer that it was. But the king was still nervous.
"Okay, I think it's ready to be activated. Your Majesty, the souls please?" - Dr Alphys requested.
The king summon the six containers in which were kept safe six human souls. Asgore then turned to Frisk, the only human who lived among monsters, and asked:
"So... you're sure you want to do this?"
"I am, Your Majesty." - Frisk replied with a sure nod - "It's our only hope for the kingdom."
"You're right, you're right…" - the looking like a goat king agreed - "I would do all the needed for it, but that you already know."
The power of six souls, with the help of the device, created a beam of light that went straight in the direction of the barrier. Frisk looked at it and sighed.
"I guess it's my turn to give the power of my soul…"
The girl closed her eyes and after concentrating for a moment, a light radiated from her chest, which soon caused the barrier to "shatter" just a little bit. A vortex appeared at that spot. Frisk smiled, relieved that their plan worked.
"You must hurry, Frisk." - Alphys instructed her - "The portal won't be open for very long."
"I know, I'm going."
"Not before a last Goodbye!" - a familiar voice said.
Frisk turned in the direction of the door and saw the queen and her adoptive mum, Toriel. She walked up to the child she loved so much, followed by Sans, Undyne and Papyrus.
"My child… please be careful." - Toriel requested as she hugged her daughter - "We'll be waiting for you."
"I promise we'll all be saved soon… I'll miss you, mum." - Frisk said as she tightened the hug, then looked at her friends - "And I'll miss you guys too."
"I suck with all the sentimental stuff…" - Undyne muttered - "But... Don't make your Journey turn out poinless! Okay?!"
"Of course, Undyne." - Frisk replied with a smile.
"take care, kiddo. that's all I have to say." - Sans said with a wink at his last phrase.
Frisk grinned just a bit before turning around to face the portal. She had to go through it before it closed. She barely made one step ahead, as she heard her closest friend's voice.
"HUMAN..!" - Papyrus said, making Frisk to look back at him - "PLEASE, COME BACK SOON. ALRIGHT..?"
Frisk gave him a warm, yet somehow sad smile. She didn't say anything, but she gave him a nod. This seemed to be enough for the skeleton, as his expression turned a bit more joyful. Frisk then turned to the portal again, ran straight to and then across it.

Storybrooke

A vortex appeared in the sea, very close to the beach. Soon, Frisk emerged from it, after what the portal closed behind her. As she walked out from the water, she immediately felt cold. She was soaking wet and the weather in Storybrooke wasn't exactly sunny. Frisk walked across the beach slowly, her body shaking.
Meantime, Regina met Henry at the bus station, as he just came back from school.
"You know I'm fourteen, mum? I can walk back home by myself." - he said, but there was no real annoyance in his tone.
"I know. I just wanted to see you." - Regina said with a smile, but her gaze betraying something else.
"What's wrong mum?" - Henry asked.
"Nothing, really. It's just… you know, after Emma and Hook got engaged yesterday, I felt like everyone's lives were moving on, while mine was standing still." - Regina confessed - "Despite this, I at least have the joy to have you in my life."
"Mum… your life isn't over." - Henry tried to reassure her as he took her hand in his - "I know, you went through a lot past months. Robin… died. You and Zelena got upset with each other."
"Angry is a more appropriate term." - Regina corrected, her tone bitter.
"But I'm sure something interesting might still await you."
"I hope you're right. And sorry about that. It was just the blues of the moment."
"It's ok, everyone has this every once in a while."
The two started to walk back home, but then they saw a figure in a corner of the street. They saw a girl standing there, shaking out of cold, her clothes wet. Exchanging a confused glance, Henry and Regina approached her, thinking that they should help.
"Miss, are you alright..? Do you need help?" - Regina asked with concern as she saw as the girl's face was growing pale.
"I... I need… to find… someone." - she replied.
"Find who?" - Regina asked.
But the girl didn't reply. She fell on the ground, passing out from the cold. Regina and Henry knelt down to her in worry.
"We need to take her to the hospital. She's freezing!" - Regina stated.
Henry didn't reply, as he was attentively looking at the girl's attire. He was shocked to see that she was wearing a blue sweater, with two horizontal pink stripes at the chest level. He immediately realized who this girl was.
"Frisk…?" - he asked in surprise.
"You know her?" - Regina asked as she looked up at her son.
"No, not really. She's the protagonist of a video game." - Henry explained - "It's called "Undertale"."
"What? I'm not familiar with the game, nor do I know a realm like this. This only means she just came from another world! Girl, can you hear me? What are you doing in Storybrooke?"
Frisk could barely open her eyes, but managed to say one single sentence.
"The Savior… I must find… the Savior…"
After she said that, she went completely unconscious, while Regina and Henry exchanged a worried gaze.

"Once Upon A Time in Underground"

Underground

It was a snowy day, like it usually was in Snowdin. Frisk was running across the woods, feeling as she was about to have a panic attack. She yelled:
"Sans! I know you're here! It's… it's me!"
Soon she felt a movement behind her and as she turned around, she was relieved to see that Sans came to her call.
"heya." - he said with a sad expression - "why are you here?"
"Sans... you know I cannot deal with this by myself…! I... I don't know how long I will be in control…"
Frisk was trying her best to not start to cry. Sans made two steps towards her. He sighed and said in a deep but sorry tone:
"it was fun for neither of us. I'm glad you're back."
"I... I..." - Frisk didn't even know what she could say as she proceeded to wipe away her tears with her sleeve - "...Something has to be done about it. Please Sans, take me to Dr Alphys' lab."
"kid..." - Sans wanted to protest, but Frisk didn't let him to finish.
"Please, Sans. If there's a solution to this, the lab is the best place to look for it."
"alright, kiddo. I'll bring you to alphys."
"Thank you, Sans..." - Frisk said with a grateful nod.
The two started to walk and Frisk was still crying quietly. Before they would use one of Sans' "shortcuts", Frisk heard a voice cross her mind.
"Don't think you can run away from me forever."

Storybrooke

Frisk woke up with a gasp. After a short moment, she sighed and looked around. It didn't take her long to understand she was in a hospital room. She then gazed in the direction of the door and could see a doctor talking with a woman she recognized as the one she saw before passing out. The said woman noticed, so she moved a bit out of Frisk's sight.
In the corridor, Regina was talking with Dr Whale, who was describing Frisk's physical condition.
"Nothing to worry about, the girl was just very cold. It seems like she did emerge from the sea, since her clothes were all wet and she smelled like salty water." - Whale said - "She doesn't seem to have any other injuries or health issues."
"So, now that she woke up, we can have a talk with her?" - Regina asked, giving another quick glance in her direction.
"Yes, but keep in mind that she might still be tired and disoriented." - Dr Whale reminded.
"Mum, why don't you inform Emma about what happened? I should be talking to her first." - Henry suggested - "She'd be more at ease with someone of her age."
Regina didn't like to delay the interrogation, but she agreed with her son. After all, they'd be more efficient with Emma around, her being the first concerned person. As Regina grabbed her cellphone to call Emma, Henry entered the room.
"Hi, do you feel any better?" - Henry asked as he sat on a chair next to the girl's bed.
"Yes... a bit." - Frisk replied a bit tiredly, readjusting her warm cover.
"As for your clothes, I believe the nurses washed them for you." - Henry reassured her.
"Good... I really love that sweater." - Frisk said.
"I bet you do." - Henry commented with a smile - "My name is Henry, by the way."
"N-nice to meet you, Henry. My name's Frisk."
"I know." - Henry said, but as he saw the girl frown in confusion, he added - "It's complicated to explain."
"When will I be allowed out?" - Frisk asked with worry - "I cannot stay here for too long, my friends need me, and I must find-"
"The Savior. You said it before you passed out. Don't worry, you'll see her soon. She's my mum."
Frisk's eyes expressed surprise, but they also lit up with hope. So, it wouldn't take long to find her and she'd be able to return to the Underground soon.
"So, your mum's the Savior? Is it the woman I saw earlier?" - she asked.
"No, that's my other mum." - Henry answered and as he saw Frisk frown in confusion again, he smiled - "That's a complicated story too."
"You haven't heard mine yet." - Frisk commented as she laid in a more comfortable position.

Meanwhile, Emma and her soon-to-be-husband were having a date, at the same restaurant they went when they had their (official) first date.
"Remember that day? Dad was so overprotective." - Emma remembered with a smile.
"Yeah, he always has been. And remember how you almost ran off from here after some thief?" - Hook teased his fiance.
"Emphasis on almost." - Emma pointed out - "I did change my mind and stayed here with you."
"So adorable." - Killian commented - "Anyway, it's nice to have a date without some enemies, criminals or other threats."
"I know." - Emma smiled.
The couple kissed and it was at that moment that Emma's cellphone rang. With a sigh, Emma looked at the screen and said with an apologic tone:
"This is Regina..."
"Can't you ignore her? Just for once?" - Hook tempted her.
"Killian... What if it's something serious?" - Emma remarked.
Hook gave up and let Emma answer the call.
"What's wrong, Regina?" - Emma asked and after a moment listening to what her friend had to say, she said - "I'll be there in a few minutes."
"So, what is it this time?" - Hook asked with a hint of disappointement.
"A video game character who needs the Savior, that is me." - Emma replied as she stood up from her chair and put her jacket on.
"What's a video game…?" - Hook asked in confusion.
"I'll explain some other day. Do not worry, I'll try to fix it soon."
"Don't make promises you're not sure to keep, love. We both know how things work in this town." - Hook remarked, but his tone was light.
Emma smiled in agreement and she hurried to reach the hospital. She arrived there within minutes and soon met Regina in the main hall.
"So, can you explain in more details now?" - Emma requested.
"Henry and I found a girl, who was freezing due to emerging from the sea, I suspect from a portal." - Regina started to explain - "Before she passed out, she said that she needed "the Savior"."
"Did she say anything else?"
"No, we brought her in the hospital while she was unconscious and I called you as soon as she woke up." - Regina answered - "Henry's keeping her company right now."
"Okay. But... what was that about the video game thing?" - Emma asked with a slight frown of confusion.
"We didn't even need to ask the girl's name, Henry guessed it right away just by seeing how she was dressed. Henry says her name is Frisk and that she's the protagonist from a game called "Undertale"."
"Undertale... Uh, sounds familiar." - Emma said pensive - "I believe Henry did mention once or twice about playing that game."
The two women were now walking up the stairs, while still talking about the odd situation. Emma seemed to be more concerned about the fact that a video game was real than by the fact someone would cross realms just to find her.
"How could a video game turn out to be real? I mean it's not even a fairy tale…" - Emma was sharing her surprise.
"Well, "Frankenstein" isn't a fairy tale either and yet, we do have Dr Frankenstein in our town." - Regina pointed out, just at the moment when they walked past the said doctor.
Dr Whale looked in their direction with an interrogative look, wondering why they were even talking about him at that moment. Regina and Emma soon reached the room, where they saw Henry sitting on the chair, waiting for his two mums, and Frisk sleeping again.
"She's asleep?" - Emma asked quietly.
"Yes, we didn't talk long before she fell asleep." - Henry replied then he gently shook Frisk's shoulder - "Frisk… Wake up, my mums came to talk."
Frisk slowly opened her eyes and as she saw the two women, she rubbed her eyes and moved to a more sitting position.
"Hi. You're name's Frisk, right?" - Emma asked gently - "I'm Emma. This is Regina. She and Henry say you want to see me?"
"Are you Henry's mother? The Savior?" - Frisk asked.
"Yes. So, tell me everything. How did you come here? Where are you from?" - Emma asked as she sat at the bed next to the girl.
"I... The story might be too confusing." - Frisk warned.
"We're quite used to that." - Regina remarked.
"You can tell us anything." - Emma encouraged.
"Okay. I came from the Underground." - Frisk started and seeing Regina and Emma's confused expressions, she clarified - "That's how the place is called. It's a world unattaignable by others, it has been this way for who knows how many years. Its inhabitants are all monsters."
"Monsters? What kind of creatures are they?" - Emma asked curiously.
"Do most of them look like anthrapomorphic animals?" - Henry asked, already knowing the answer.
"Y-yes... How did you guess?" - Frisk asked, confused on her turn.
Emma knew it wasn't the best moment to start to explain that there is a video game based on the world, because it could confuse the girl, or even freak her out. So she hurried to ask her own question, encouraging to tell more of her story.
"But how did you get there in the first place? I mean, if it's a world of monsters, there shouldn't be humans around?"
"I'm originally from New Jersey." - Frisk said - "I ended up in the Underground by accident. Or by fate, who knows."
"Wait, you're saying that you're from this world? You should be relieved to know that you're back in the USA." - Emma informed her, which seemed to reassure the girl.
"This is good to know… Anyway, the Underground eventually became my home. Its inhabitants became my friends, my family… Toriel, the queen of the monsters, became like a mom to me." - Frisk confessed - "So... I can't stand the thought that they have to be trapped, that they're in danger… I just want them to be truly happy."
"So, what's going on exactly? What do you mean by saying they're trapped?" - Regina asked.
Henry resisted the urge to start explaining the plot of the game. Frisk would be a better storyteller, actually being part of that world and having a true inside look.
"Centuries ago, the kingdom of monsters has been trapped by human sorcerers. Humans didn't trust monsters, because they feared their powers, their bodies being made of magic. Though, there was no reason to fear monsters, because they paradoxally are… "weaker" than humans. Humans have an immortal soul, while monsters' souls disappear fast after death."
"This is sad..." - Regina said to herself.
The mention of souls disappearing reminded painful memories about Robin Hood's death. But she did her best to ignore the sharp pain that crossed her heart and to focus on the rest of Frisk's story.
"So what happened is that monsters' kingdom got sealed away from everyone's reach. No one can access it, from neither world. In theory, that is."
"Yeah, you did end up in the… Underground somehow." - Regina commented - "How is it possible if their world is supposed to be unattaignable?"
"Every once in a while, there are… "cracks" in the barrier. I suppose it's because of the prophecy. That an "angel" would fall and who'd free them all. Someone who'd restore contact between monsters and humans." - Frisk explained.
"It was about you?" - Henry half-asked, half-stated.
"I'm not sure anymore." - Frisk shook her head - "When I landed in the Underground, all I could think of was to return back in the world of men. But throughout my journey, I grew attached to monsters as I learned to know them. Helping them became my goal and in the end, I succeeded. The barrier surrounding their world was broken and the monsters were free to travel to other worlds again. But…"
Frisk grew silent. She wasn't sure she could keep going without crying. Seeing the girl's expression, Emma took Frisk's hand in hers and encouraged her to pursue:
"Take your time. Whatever the problem is, we'll help."
Frisk nodded, thanksful to the support, and pursued her tale.
"There's… there's one thing I didn't mention yet. When I arrived in the Underground, I discovered that I had some sort of a... special power."
"What kind of power?" - Regina asked in interest.
"A power to… come back and fix things." - Frisk replied, but it obviously wasn't clear to Regina and Emma - "The place was obviously dangerous, many monsters at first wanted my death. I almost died so many times… But the power I have permitted me, as I were in a state between life and death, to go back to before my death and do things a bit differently to not die. I'm not sure if it's clear..?"
"No, I think I understand now..." - Regina said pensively.
"But later, I discovered that I could use that power willingly any time I wanted, not only when dying." - Frisk pursued - "This permitted me to fix a mistake."
"A mistake?" - Henry asked.
"Yes. You see, I... I always showed mercy to monsters, even when they tried to hurt me. Then, there was Toriel who wanted to keep me locked in her house, to keep me safe from Agore, the king. As I insisted to go out and try to get back home, she blocked my way and dared me to prove her that I were strong enough to survive outside. She engaged a battle with me. I didn't want to fight, but I needed to defend myself from her attacks. Then… I... I'm not even sure how it happened… but..."
"Oh no..." - Henry already guessed, remembering his own first experience with the game.
"I must have hurt her in some way that... Maybe she's a fragile individual… I just remember how she fell on the ground in pain and… said some last words before… turning to dust. I could see her soul a few moments before it shattered. I... killed her… But I didn't mean to..."
Frisk then wiped away a tear that was streaming down her cheek. Emma, Regina and Henry felt sorry for her. It must have been some shocking experience.
"Hey... But now she's alive isn't she? Is that the mistake you fixed?" - Emma asked.
"Yes... I used my power to go back and not kill her." - Frisk said, managing a smile.
"But so, you say everyone was alive and well in the end. Why are they still in danger?" - Regina asked.
"We all lived happily for some time. But... it looks like something beyond my will just didn't want us to have that happy ending." - Frisk said with sadness - "I'm not sure yet who's to blame, but all I know that once day, I returned back to a point before the end of my journey. The power to go back had been used. But... not by me."
A silence followed as Regina, Emma and Henry processed. Henry was the first to break the silence, asking:
"But... why didn't you just do the same thing again?"
"I could have, but there was no point in that. If something made me go back, this can only mean that my victory would be short. I understood that I had to do something differently this time."
"What did you do then?" - Emma asked.
"I... before facing the king Asgore, I decided to share my knowledge with someone who, as I figure out, has the ability to remember the previous… timelines?" - Frisk attempted the terminology - "I shared my concerns and confusion, we then talked to the royal scientist, who managed to convince Asgore to leave me alone. We then decided to try make more research, to figure out a new solution."
"But... how does it all have to do with me?" - Emma asked, still a bit confused.
"I was coming to that part. I found in Dr Alphys' lab some old notes and entries made by the previous royal scientist." - Frisk replied, her tone getting serious - "In one of them, there was a mention of a "Savior", the person who has the power to give everyone's happy endings."
"I see now... so, that's why you went after me." - Emma understood - "You think I'm the key to your ultimate happy ending."
Frisk just nodded. Emma and Regina looked at each other and decided that they should let Frisk rest, while they would start to study the situation that was brought to them.

Underground

"Dr Alphys!" - Frisk yelled as she ran through the doctor's lab corridors - "I think I have something!"
"Frisk? What is it? Something good?" - the yellow dinosaur-like monster asked with nervousness.
"It is very good!" - Frisk answered with a big smile of hope on her face.
She showed to Alphys the translated to English entries of the previous scientist. The guy seemed to write in a language of symbols, but thanksfully Alphys had the technology to scan and translate the entries written in symbols. Frisk spent most of her time trying to find something helpful in those entries and finally came the moment when she did. Frisk took a pen and underlined one phrase in particular, and Alphys read it out loud:
"Dark, darker, yet darker… But I see a special person, a Savior, bringing back the light."
After a moment, Alphys looked up at Frisk and commented:
"Sounds relatively prophetic."
"I think it is a prophecy indeed." - Frisk agreed - "Our next step is to find that said Savior and bring them in the Underground."
"What?! No, it could be dangerous…" - Alphys protested, getting nervous
"Nothing'll be more dangerous than who we must face." - Frisk remarked seriously.
"But the doctor didn't leave any clues about that Savior. They could be anywhere…!"
"There must be some clues we missed in the doctor's other entries. We just must go through it again." - Frisk said calmly - "Come on Dr Alphys, don't you want to save us all too?"
"Alright... Let's do it for our kingdom." - Alphys nodded with a smile.
The two took a pile of translated entries and proceeded to study them once more.

Storybrooke

Frisk was allowed out of the hospital and Emma decided that she should let her sleep in the guest room of her and Hook's house. As Frisk entered the living room, she met Hook, who was sitting on the couch. He looked up at the girl and he said with a smile and fake reproach:
"So, you must be the one who stole my future wife in the middle of our date."
"Sorry." - Frisk said with an embarrassed smile - "You must be Henry's father?"
"N-no, I'm not the kid's dad." - Killian replied, then added with a wink - "But I hope the same won't be said about her next one."
"What were you talking about?" - Emma asked as she joined them.
"Nothing." - the two replied at the same time, Hook's face impassive while Frisk was grinning.
"So Frisk, follow me, I'll show you the room for tonight." - Emma instructed.
Frisk followed Emma up the stairs and the woman showed her the room and gave her a blanket.
"Sorry, the room's a bit... empty."
"It's okay. It's just temporary anyway." - Frisk shrugged.
But she felt sadness as she remembered how Toriel gave her a very colorful room with many fournitures. Frisk wasn't gone for a long time yet, but she already missed her goat-mom. She guessed it was because she couldn't have news about her or others from where she was.
"Okay then, I'll leave now. Tomorrow's gonna be a long day." - Emma commented.
She was about to leave, but Frisk stopped her with a question.
"Can I ask you a favor...?"
"What is it?"
"Could you please... lock me from the outside?" - Frisk requested.
"What..? Why? You're in no danger here." - Emma stated confused - "No one will hurt you, especially not in my house."
"That's not what I'm fearing." - Frisk said.
Emma was surprised by the behavior, but decided against pressuring her with questions. She guessed Frisk would tell whatever bothered her when needed.
"Ok. If it makes you feel any better, I'll put a protection spell on your door." - Emma said.
"Thank you." - Frisk thanked her with a small sigh of relief.
Emma did as Frisk requested her. As she closed the door, she put a spell over it, so no one could cross it from inside or outside. Frisk laid down in the bed and sighed as she closed her eyes. The next day would be a difficult one, since they'd have to figure out the way to return to the Underground. How long would it take? Was Emma strong enough to deal with the greatest threat to the Underground? It was with a mind filled with questions and worries that Frisk drifted into the sleep.

A/N: So, what did you think about the first chapter? Please, let me know. And if you have any questions, please don't hesitate to ask. I'll gladly answer, unless it's too spoiler-ish. ^^

For those who follow my "Once Upon A Light" story, don't worry. I'll keep writing that story too. It's not a project I plan to abandon ever. I hope I can update it soon.