(A/N: IMPORTANT! PLEASE READ! Hey there, good people, and I'm glad to see you managed to find this new story of mine. I say that because it's a crossover, and crossovers don't always get that much notice. Even I hardly ever write crossover stories, having only done 7 so far, excluding my Death Battle stories, despite having written over 100 stories. This time though, it's a three-way crossover involving Amphibia, The Owl House, and Gravity Falls. All three of these shows are canonically confirmed to take place in the same universe, and ever since hearing the rumors that there has been talk of a movie featuring a three-way crossover between these shows, I've been borderline obsessing over the idea, and that's where this story came from. Now, seeing as FFN only allows crossovers involving 2 series, I decided to list this one under Amphibia and The Owl House since those two shows are more closely related, but it's also equal parts Gravity Falls. If I could have included a third series, the four characters this story would have been listed to be about would have been Anne, Luz, Dipper, and Mabel, but seeing as I can only use two, I went with Anne, Sasha, Luz, and Amity. I'm working with what I've got, ok. But the reason I requested that this Author's Note be read is for an important plot point in this story. So, as you may or may not know, there was a canon crossover script reading where Luz, in attempting to create a portal to the human world, accidentally created a temporary one to Amphibia instead, where she met Anne and the Plantars. That script reading will be addressed in this story as a major reason for some of the characters meeting. So if you haven't seen the script reading, I recommend checking it out. It's only 10 minutes, and you can find it on YouTube. It even has fan-art to go along with it so you have more than just audio. I'll post the link to the video at the end of this chapter. While I will address it in the actual story, I still suggest checking it out before checking out the next chapter, just so you're properly informed of how Luz and Anne know each other. But for now, enjoy the first chapter.)

Disclaimer: I own nothing.

"THREE SIDES TO THE TRIANGLE"

Chapter 1: That Triangle Guy

Marcy Wu loved lucid dreaming. It was like living through a fantastic adventure. It was even better when she was able to control her dreams, allowing her to do the impossible. Kind of like what was happening right now.

Of course, being the brainiac she was, she'd done research on how to recognize when she was dreaming, how to seize control of her dreams, and how to better remember the events of her dreams after she woke up. There were several different ways in which she was able to achieve these goals, and she tried as many as possible, until she found what best worked for her.

And it did work. Granted, not all the time, not even most of the time, but more often than before, she was able to realize that she was dreaming. And with some mental effort, she was able to control certain aspects of her dreams. Flying in particular was a favorite of hers, and she was currently soaring through the depths of space, accompanied by dream versions of her closest friends, Anne Boonchuy and Sasha Waybright.

Sometimes Marcy wondered if her time connected to the Core contributed to her being able to recognize when she was dreaming and controlling what happened in them, but she pushed such thoughts away. She'd rather not think about the time she had basically been possessed by the machine that had contained a collective conscience consisting of the most brilliant of minds and leaders of Newtopia uploaded into a super computer over the past few millennia so they could essentially live forever and continue their conquest of different worlds across the multiverse.

Once again, she pushed such thoughts aside. That had been several years ago. They had stopped the Newtopian invasion of Earth, Sasha had freed her from the Core, and Anne had destroyed it once and for all, finally putting an end to it and its reign of terror and tyranny for good. It was just a shame that they'd had to say goodbye to their amphibian friends after that when they returned to Earth. And with the destruction of the Calamity Box, particularly its three magic gems, there was no way to get back to Amphibia.

A smile crossed Marcy's face. She may not be able to see her amphibian friends in real life, but she could see dream versions of them here in the dream world. And so, with some concentration, she manifested them flying beside her. The frogs Sprig, Polly, and Hopediah Plantar appeared beside Anne while the toad known as Grime appeared by Sasha. And beside Marcy, the newts Olivia, Yunan, and King Andrias popped into existence. Yes, she still considered the former king of Newtopia to be her friend, despite his betrayal; he had turned against the Core and sided with them in the end after all.

"Who-ha-ha, we're flying!" Sprig exclaimed joyfully. "This is awesome!"

"I can fly faster than you!" Polly declared challengingly as she zoomed past him.

The pink-frog smirked as he sped after his sister. "We'll see about that!"

"Kids, you get back here!" Hop Pop scolded, and attempted to swim through the air to fly after them.

A competitive sneer spread across Grime's face. "If you think I'm about to let a frog outfly a toad, you've got another thing coming."

As he sped after the two of them, Sasha cupped her hands over her mouth and called after her toad friend. "Woo-hoo! Go, Grimesy!"

"Sprig! Sprig! Sprig! Sprig!" Anne cheered, pumping her fists in the air each time. "Oh, and Polly!"

Next to Marcy, Lady Olivia rolled her eyes. "Oh, for goodness' sake, such childishness."

On Marcy's other side, King Andrias let out a hearty laugh. "Come now, Lady Olivia, learn to have a little fun."

An affectionate smile spread across Marcy's face. She really wished they could all be together like this, but alas, it was not to be. In fact, she had no idea what her amphibian even looked like right now after all these years. Dream Anne and Sasha resembled their real-world counterparts, as did she, but the others maintained the appearances they had when she last saw them, and that had been ten years ago.

But beggars couldn't be choosers, or so the saying went. She was lucky to be able to hang out with the real Anne and Sasha again. In the real world, she had finally managed to reunite with them after she and her parents had moved to Massachusetts. An event that, ironically, had kicked off their whole adventure in Amphibia, even if that had been mostly unintentional on Marcy's part. But at least they could all be together like this in her dreams.

But then their deep space dream adventure came to a sudden end, quite literally. As Sprig, Polly, and Grime flew on ahead, they suddenly crashed into an invisible wall. Or perhaps it wasn't so much an invisible wall as it was that the further depths of space were revealed to be nothing more than a screen.

As the others caught up, Marcy tapped her chin as she examined the barrier. "Hmm, quite interesting. It appears to be a wall that perfectly reflects our path ahead. Quite curious." But then her face lit up. "Or not. This is a dream after all, so anything can happen here."

As soon as she said this, a loud echoing laughter erupted seemingly from all around them. It was a sadistically gleeful laugh that bordered on deranged, and Marcy and her friends huddled together uneasily.

"Ooook, what's with the strange laughing?" Sprig asked nervously.

Polly just smirked. "I like it. It gives a sense of doom."

An uneasy feeling suddenly fell over Marcy. "I know that laugh…"

All of a sudden, a blue flame appeared high above them. Streams of fire flew outward, forming a ninety-degree angle. Then the two ends came together, forming the flaming outline of a triangle. All the stars with the triangle suddenly came together, forming a ball of light in the middle of it. This ball of light opened like an eyelid, revealing a giant eye. Cracks then spread across the dark surface of the triangle before it shattered, revealing a golden yellow form underneath.

"Hey, Marcy, did you miss me?"

Marcy jolted back. "You again."

She looked up at the… creature, for lack of a better word. It was a giant glowing, golden triangle with a single giant eye in the middle, very akin to the Illuminate symbol, only this triangle had a black top hat that seemed to be a part of its body, a black tie beneath its eye, and black stick-like arms and legs. It could pass for cute if it didn't give off such a sense of foreboding.

"Me again," the triangle replied, its whole body flashing as it spoke without a mouth. "I see you're enjoying a little space flight with your friends. Too bad they're not real."

The triangle snapped its fingers, and Marcy's friends all let out silent screams before distorting and fading away. The triangle laughed as it and Marcy were left alone.

"If you're wondering why they didn't make any sound," it said almost mockingly, "it's because in space, no one can hear you scream. A shame really, screams are hilarious!"

A low growl escaped Marcy as she glared at the triangle. This hadn't been the first time she had seen it in her dreams. He, at least she thought it was a he based on its tone of voice, had started appearing a few months ago, and had been gradually appearing more and more often. Strangely, he didn't feel like he came from Marcy's mind, but rather was something… foreign.

"Uch, why do you have to constantly ruin my fun?" she complained irritably. "And why can't I just dream you away? I know you're not real, so just leave me alone."

The triangle nonchalantly began fiddling with his fingers. "I may not be real now, but things change, Marcy Wu. Things change." His voice grew impossibly deep and distorted when he said "change", his color briefly flashing red. "Every day I grow stronger, and soon I'll be strong enough to be more than just a part of your mind. So enjoy normalcy while it lasts, Green Gem, because Bill Cipher is coming back."

He dissolved into more deranged laughter, and as he did, Marcy felt her dream ending as everything began to fade around her. A moment later, she let out a startled gasp as she jolted upright in bed.

She breathed heavily as the remnants of her dream affected her, but then let out a sigh as she recognized the somewhat familiar setting of the hotel she was staying at, having come back to Los Angeles for a reunion with Anne and Sasha a few days ago for the former's birthday. She wiped her damp forehead as she reassured herself that what she had seen was nothing more than a dream, no matter how real it felt.

"That stupid triangle guy again," she grumbled irritably. "Why do I keep dreaming about him?" She tapped her chin thoughtfully. "Hmm, maybe I should look up dream interpretations and see what a triangle represents."

She grabbed her phone off the nightstand, intending to do just that. However, the moment her screen flashed to life, her eyes widened in shock.

"Is that the time!?" she exclaimed. "I'm supposed to meet Anne and Sasha!"

Throwing off the covers, she ran into the bathroom to relieve herself. She then quickly washed her hands and face before snatching up her toothbrush, deciding she could grab a bite to eat on her way to her destination.

But as she lifted her toothbrush to her mouth, she caught sight of her reflection in the mirror. The face she was making was different from the one that should be reflecting. Her mouth was stretched nearly ear-to-ear in a deranged grin, her eyes wide and having taken on a yellowish hue, and her pupils had elongated and become wavey. The sight caused her to cry out and she dropped her toothbrush as she jumped back in surprise. As she did so, she could have sworn she heard the sound of Bill's laughter.

Taking a few deep breaths to calm her racing heart, she carefully moved in front of the mirror again. Much to her relief, her own normal reflection greeted her. She shook her head and rubbed her eyes before taking another look, but nothing had changed. It was the same face she always saw when she looked in the mirror.

Taking a deep calming breath, she let it outing an exhale. "Get it together, Marcy. It's just your mind playing tricks on you because of that weird dream. You're still half asleep, that's all."

She told herself that, even though she felt wide awake. She had also felt wide awake before she saw her freaky reflection, the last traces of sleep having disappeared when she saw the time. Which reminded her.

"And late!" she exclaimed, grabbing at her head. She quickly began looking around. "Toothbrush! Toothbrush! Where'd I drop my toothbrush?"

She froze as she spotted it in the toilet beside the sink. She stared at it for a few moments before her shoulders sank in dismay. "This is going to be one of those days, isn't it?"

(-)-(-)-(-)-(-)-(-)-(-)-(-)-(-)

"Mar-Mar, you're late," Anne stated as Marcy joined her and Sasha.

Next to her, the blonde girl with a scar on her cheek crossed her arms and smirked. "Stay up too late playing a video game?"

Marcy bashfully rubbed the back of her head. "Sorry, sorry. Had a bit of a rough start this morning. Been having these weird dreams about some triangle guy." She then cut her eyes at Sasha. "And I don't get as lost in my video games as I used to."

The blonde girl just chuckled. "I'm sure. Never change, Marcy. Never change."

Marcy rolled her eyes and turned to her Thai friend. "You still have time to give us an exclusive tour though, right?"

Anne beamed. "Of course. I'm not actually working today. Come, allow me to bring you through the wonders of Amphibia."

This Amphibia, however, was not the amphibian world in another dimension they had gotten lost in for months. Rather, it was the name of the reptile house at the Aquarium Of The Pacific where Anne worked. It had been an unexpected career choice for her, especially since she had found creatures like frogs to be repulsive. Of course, that had been before they had spent months in Amphibia and she had come to love the world and its people. So perhaps it wasn't such a strange career choice for her after all.

But Anne didn't just work at the Wonderful World Of Frogs section of the aquarium, particularly with the exhibit dubbed Discover Amphibia, she'd really helped make it what it was today. As such, she had certain privileges as a prize employee, and so had been granted permission to give her friends an exclusive, behind the scenes tour, showing how she took care of all the different critters.

"Did I mention how I think it's really great that you're doing this?" Marcy asked as she looked around from where Anne had brought them in the backrooms. "Seriously, a herpetologist. Our time in Amphibia really had an impact on you, didn't it?"

Anne chuckled. "Yes, you did, and yeah, it did."

"I think it had an impact on all of us," Sasha replied. Her hand subconsciously came up to touch the scar on her cheek. "For the better."

An air of melancholy fell over the three of them as they thought back to their time in the alternate dimension. While each of their experiences had been very different, they had helped them to grow into the people they were now. And both Anne and Marcy could agree with Sasha that it had definitely been for the better.

"You think we'll ever go back?" Marcy asked quietly.

A sigh escaped Sasha. "I don't see how we can, Mar. The stones were destroyed, so we have no way of getting there. I mean, I know Terri said that she and her team are making a lot more progress with their dimensional doorway project, we saw the results ourselves a few days ago, but creating a portal specifically leading to Amphibia still seems a long way off, which is a real shame." Her shoulders slumped slightly. "It would be nice to check in on Wartwood to see how everyone is doing, and I do miss old Grimesy."

The mood shifted from melancholic to more gloomy as they thought about their old friends. Sasha and Marcy knew that Anne must especially miss the Plantars since they were literally her adoptive family in that world.

Deciding to look on the positive side, Anne gave her friends a smile. "Hey, it's not like it's impossible. After all, we opened up a portal to it without the stones before; that's how the Plantars and I got back. And like Sasha said, Terri's working with Mr. X and Dr. Jan to help build a new working portal. Ally, Jess, and Molly Jo are helping however they can too. So, who knows, maybe they'll actually be able to pull of creating a stable portal one day and we can go visit them whenever we want."

Marcy chuckled. "It's a nice thought, but even if they did manage to pull it off, I doubt we'll be allowed free access to the portal like that. Mr. X does want to keep it hidden from the government after all, so it's not like we can just casually use it."

Sasha shrugged. "Eh, you never know. I mean, Anne and I did save the world from an invasion. Maybe we could be ambassadors or something. And don't forget, Anne is destined to become the new godly guardian of the multiverse one day."

Anne winced at that. "Yeah, I still haven't agreed to that. And I'm still thinking no. The gig sounds a little too much for me, no matter what perks there are. The current guardian even admitted that it's an exhausting job, and that thing's a deity."

Smiling, Sasha playfully wrapped an arm around Anne's shoulder. "Ah, I'm sure you could handle it, Boonchuy. But hey, you still have several decades to change your mind. How long did the guardian say you were going to live to again? Ninety-one, was it?"

Anne winced again and made a hissing noise. "Yeeeah, I try not to think of my life expectancy. It's almost like counting down to the date of my death. Do you know how freaky that is?"

Sasha just playfully tightened her hold around Anne's neck. "Hey, don't worry about it so much. I mean, you died once already and came back."

Anne's pupils shrank to pinpricks in her eyes. "I still face existential dread over that," she said in a dramatic voice.

Removing her arm from around Anne, Sasha slapped her on the back. "Come on, it's not so bad. Just look at it this way, when you die, you either take the gig and become this super powerful, multi-dimensional guardian until you find your own successor and retire, or you turn it down and join the rest of us in the afterlife. Either way, it's a win-win, right?"

Anne raised an eyebrow at her, as the choices Sasha had basically presented her were to either stay dead when she died or take on an exhausting job for an undetermined amount of time. "I… guess… When you put it that way…"

"Atta girl. Now come on, show us your mini-froggy friends."

Marcy's face lit up. "Yeah, I love how you made the frog section a replica of the Plantar farm, the toad section Toad Tower, the newt section Netwtopia Castle, and the olm section Proteus. And didn't you say that you have a frog that looks like Sprig?"

Grateful for the change in topic, Anne smiled. "Oh, yeah. That's why I named him after him. Sprig I mean. He's over here." She stepped over a puddle by one of the exhibits. "Oh, but watch you step, we got a leak there. Gotta get that fixed."

Taking out her keys, she unlocked the exhibit and reached inside. When she pulled her hand out, there was a little pink frog sitting on her finger. Smiling, she turned to her friends, holding the frog out for them to see.

"Oh, he's so cute," Marcy cooed, her eyes large and shining.

Sasha snickered. "He really does look like Sprig, just without the hair. Or the standing on two legs."

Anne nodded. "Yup, I like to think that he's this world's version of Sprig."

Knowing Anne's love for shipping, Sasha smirked at her. "Maybe you can find one that looks just like Ivy and see if you can get the two of them to hook up. They'll be your own personal Sprivy."

Just as she suspected, the look that appeared on Anne's face suggested that a whole new world had just opened up to her. "I now have a new purpose in life," she whispered in awe.

Marcy giggled. "Anne, you're so predictable sometimes." She looked back at the pink frog, reaching out with her finger to touch it as she spoke in a baby voice. "Would you like that? Would you like that, little guy? You want Anne to find you your very own Ivy?"

Almost as if in answer, the frog leapt off Anne's finger and right onto Marcy's face. A startled cry escaped her and she stumbled back, waving her arms. She ended up stepping in the puddle Anne had warned her about and slipped. It seemed that even in adulthood, she was still a klutz.

"Marcy!" her friends cried out, reaching for her.

It was too late to stop it though and she went down, hitting her head hard on the ground. Her friends both winced with Sasha letting out a hiss and Anne an "Ooh". The latter quickly snatched up Sprig as he hopped away and stuck him back in his display before shutting the door and dropping down beside her fallen friend, Sasha crouching down in her other side.

"Marcy, are you ok?" Anne asked worriedly.

"That was a hard knock," Sasha commented, looking as if she felt her friend's pain.

A groan escaped Marcy, but she appeared to be out of it. Anne looked around in concern before picking up Marcy's arm and draping it over her shoulder.

"Come on, let's get her outside," she suggested. "Maybe some fresh air will help clear her head."

Taking Marcy's other arm, Sasha dropped it over her own shoulder, and they hauled her to her feet. But as they did so, a thought occurred to Sasha. "Wait, aren't we not supposed to move someone with a head injury?"

"I don't know, I'm a herpetologist, not a doctor," Anne replied.

"Yeah, but I think that's supposed to be common knowledge."

A frustrated growl escaped Anne. "We already got her up, let's just take her outside and go from there."

Working together, they managed to carry outside and set her down so she was propped up against the wall. She appeared to be out cold, her head hanging down limply. Anne carefully felt the back of her head and winced as she felt the bump forming there, but at least there was no blood.

"Well, it doesn't feel too bad," she commented. "I think she just knocked herself out cold."

Sasha sighed and shook her head. "Classic Marcy, still a klutz."

Reaching for her belt, Anne took up her walkie-talkie. "I'm going to contact the medics. Probably should have been done that before we moved her." She switched on the walkie-talkie. "Dispatch, this is Anne Boonchuy, requesting medical assistance."

Before she could say anything more, there was the sound of joints cracking as Marcy's head turned to the side. Anne's attention returned to her friend as Marcy's head turned in the other direction, emitting another cracking sound.

"Dispatch to Boonchuy," the walkie-talkie responded, "what is the nature of your situation?"

Not taking her eyes off Marcy, Anne hesitantly replied, "Hold, please."

Marcy's head turned again, and this time her arm jerked as well. More cracking was heard as Marcy's arms and legs began moving before she slowly lifted her head. Her eyes remained closed, but a grin that was a little too big and slightly off spread across her face. Then her eyes suddenly snapped open as an unsettling chuckle escaped her before she spoke a voice that was not her own.

"Hello, world. Did you miss me?"

(A/N: And there we go, the first chapter. When I first heard the rumors of the three-way crossover movie and that it would feature Bill returning as the villain, I began brainstorming ideas of how, and that's what sparked the idea for this story. It seems that Bill has somehow managed to return, and get into Marcy's head. And I'm sure you can probably guess what happened at the end there. Don't worry, I'll explain how Bill is back and got into Marcy's head in the next chapter. But like I said at the beginning, I suggest checking out the Owl House x Amphibia crossover video. It's not a REQUIRED watch to understand what happens, but it is an entertaining video that will provide more details of some plot points in this story, and as I said, it's only like 10 minutes long. I've posted the link to the video below for you to check out, just replace the DOT with an actual period. But whether you check it out or not, let me know what you think of this first chapter, and get hyped for more, because there's a lot coming.)

youtubeDOTcom/watch?v=HXubFVwRp8Y