Well, since Gravity Falls ended—and I'm still trying to get a move on after a year—while Star vs the Forces of Evil is still on hiatus, I thought I could write something to… I don't know, get my life going? I just want to write a crossover of these two shows, okay?

Gravity Falls belongs to Alex Hirsch

Star vs the Forces of Evil belongs to Daron Nefcy

…

So, here's the full intro of the story…

Marco Diaz met a magical girl when he was fourteen years old. They became best friends… and soon, he became secretly in love with her. For two years, they hung out together, fought evil monsters and enjoyed each other's company. But before telling her how he really felt, Marco watched Star return to her home dimension.

Sure, he tried distracting himself with different girls like Jackie but Marco never let go of Star. After three years of waiting, Marco becomes desperate and tries to take her back.

He discovers a way to do so by using dark arts. While walking home after hanging out with Janna, a stranger gives Marco a scroll about summoning a powerful demon that grants any desire with a price. Marco decides to try the summoning and it actually worked…

...

Enjoy!

PROLOGUE:

The Deal

Marco was still confused when his room was suddenly transformed into an empty void filled with multicolored lights. But the creature in front of him was the most confusing thing in the whole room.

"Well, kid?" The creature asked again in a tone Marco knew as interest.

This thing is interested in my thoughts?

"I… uh…" Marco stammered.

"Ugh, listen karate kid, this is a once in a lifetime opportunity, don't let go of this chance!" The creature floated around Marco like a butterfly inspecting a flower.

Butterfly…

"Kid, doing business with me is an honor… I don't know if it would last though…" If the thing has a mouth, Marco knew it would be smiling at what it just said. "The offer… I meant the offer … not the actual deal."

"Why are you interested with this kind of deal with me?" Marco asked, stopping the creature's circling. "I mean, I'm desperate but you're making more than desperate…"

"Oh, boy…" The creature snickered. "Okay, you're suspicious of my intentions and I can respect that. I'm a demon for Pete's sake! But let's not forget the very reason why I'm now here talking to you…"

Marco looked at the creature's eye: a singular sphere with cat-like pupils that somehow radiated fear that reminds Marco of his darkest nightmares.

"I was trying to get her back—"

"So you summoned me to attend to your problem! That's the very reason!" The creature began to float around Marco once again. "You lost someone and you want to have her back, right? And that someone is—or should I say was? I'm confused—very special… someone close and someone… oh, now this is getting interesting…."

The demon made a sound like it might be a purr of pure interest.

"Stop talking demon!" Marco yelled.

The creature stopped floating again and settled in front of Marco. The thing's eye stared at Marco starting from his feet all the way up to his face.

"Tell me karate boy, did she die?"

"No! She's alive—"

"But she's not here! Then where is she?"

Marco felt his face burning in fury. He closed his hands into a fist. His teeth are clenched in anger.

Just accept… The demon's voice mused inside Marco's thoughts. She's alive but…

"She… she went home…." Marco finally said. His anger was suddenly replaced by sadness. "She left Earth to fulfill her duties to her home… she… she moved on…"

Marco's knees buckled. He collapsed on the floor and began to sob. This of course would be very embarrassing, being a black belt in karate. The twerp, Jeremy Birnbaun would be laughing if he sees Marco like this. But now, Marco doesn't care. All he cares is finding a way to get to the girl she lost three years ago…

… The girl he loved…

"Star…" Marco sobbed.

A moment of silence followed but was broken by slow claps. Marco looked at the creature who is now seated in a red recliner that appeared out of nowhere.

"Now, that's a show!" The demon cackled. "I should've brought my camera. Oh, wait, my eye is basically a demonic camera! Ha! Ha! Ha!"

That got to Marco's nerves.

"Shut up you-you…" Marco stood up and walked to the demon.

"Go on, karate kid!" The demon asked in amusement. "What am I?"

"You're a stupid… nacho demon!"

The chair disappeared. The demon returned to its floating.

"Okay, being called a Dorito was an insult but a nacho? Wow man, you should really watch the stuff you use. I mean, your girlfriend and you used to eat nachos together, right?"

That caught Marco's guard.

"She's not… really my girlfriend…" He muttered under his breath.

"Man, this is taking so long…" The demon floated around Marco once again but after doing so, it sighed. "Listen, I may be a demon but I play fair… most of the time, to be honest, but I know you want her back and I know her identity as a royal blood next in line to a throne made your love life suck…"

The demon stopped abruptly only to release a small laugh.

"I can help you with that…" The creature's eye blinked and suddenly the pupil vanished, replaced with a scene. It was like looking at a security camera recording but it showed Marco himself with… Star Butterfly. "I know there were so many times you tried to tell her how you really felt but you were afraid it might destroy your friendship… you wanted to be more but you were afraid to try because it might shatter your relationship…"

"'How did you…" Marco's eyes are still fixed on the scenes shown in the demon's eye.

"Know these? Well, okay, I guess I could tell you…" The demon scratched its triangle's top which could be interpreted as the head. "Kid, I see all! I watch everything! I see all dimensions and realities! Dreams and minds of every living thing: I see 'em all!"

Marco is now very disturbed. What kind of demon is this?

He focused on the new scene in front of him: it was him and Star, dancing. It was the night of the Blood Moon. Marco remembered Star's red dress while he wore the traditional Dia de los Muertos outfit. He remembers the memory clearly: the dance, Star's expression—especially her eyes—and the red light of the Blood Moon.

Sure, the night nearly ended with his death by the hands of Star's ex-boyfriend, Tom but thanks to Star, Marco didn't die from the demon boy's wrath. And now, Marco is wondering if Tom and the demon in front of him are somehow related.

But the point is: the dance surely did something to the two of them. Marco felt more connected to Star than ever. Did they really become soul mates that night? How much more does this demon know?

"Ah, the dance that bonded you forever as… soul mates…" The creature mused. "Funny, I could use a soul right now!"

With one last look at the dancing scene, Marco took a deep breath.

"Okay..." He muttered, enough to attract the triangle's attention. "I'm ready to take her back."

"Ah…" The demon's eye blinked and returned into the narrow pupil. "Are you sure? I mean, she's already happy in Mewni… and you want to ruin her present life?"

Marco looked at the demon with disbelief.

"Are you kidding me? I thought you want me make the deal with you! Now you're telling me that it's not a good idea? What do you want?" He yelled.

"Hey, karate kid, it's not what I want that matters… it's what you want…" The demon shrugged. "Have her back? Or… let her live her life? I'm not kidding that she's having fun with her life in Mewni… I mean, she's a princess and people love her! Heck, she's even got a hundred suitors to choose from to be her one and only! But again, I know you still want her and you want her to be with you… so the choice is really yours… not mine…"

Marco looked at the floating triangle demon.

She's happy… but you're not…

Marco flinched. It wasn't the creature's voice: it was his own.

Choose Marco Diaz… choose…

Am I getting crazy?

No, you idiot! I'm you! And think about it! What do you want? Be a sad sack forever until you die a lonely old crap? Or be with Star and live life to the fullest?

I'm not greedy!

"Oh come on kid… I know you have greed inside ya!… everyone does!" The demon sighed like it heard the whole conversation inside Marco's head. "Three years ago, there's another kid who made a deal with me because of his desperation not considering it was a selfish action! But… the deal didn't go well for me… stupid Pine Tree…"

Marco doesn't know whatever the demon said but the considering part caught him.

She's happy, I'm not… if I leave her where she is… she'll live her life and forget about me…

…

But if she's with me I won't be sad… I'll make her happier…

"Well, kid? Time's slipping! Is it a deal? I ain't got all day!" The demon stretched out its hand. "Shake my hand, karate kid to seal the deal and it will be done!"

Seal the deal…

Marco thought of the times Star made him smile and the times they shared in countless dimensions… her laugh, her smile… the girl he ever loved…

"What's in it for you anyways?" Marco glared at the demon who lowered its arm. "You never mentioned what would be your benefit. So, what is it?"

"Well, aren't you a cautious one?" The demon laughed. "Don't worry; my benefits come from the benefits of my clients. I get their… scraps… did that answer your question?"

"Not entirely." Marco admitted.

"Well, I'm mystified as you are, kid. I don't know my benefits unless you seal the deal with a handshake." The demon shrugged again. "If you don't believe in me, then don't! You're the one who summoned me in the first place! And I'll repeat: this is only a one chance opportunity!"

Marco looked at the creature once more.

"Well?" The demon stretched out again, waiting for Marco's handshake.

Go on, Marco… seal the deal… get Star back!

Marco hesitated.

What are you doing?

"Doubts, karate kid?" The demon chuckled. "Come on, make up your mind already!"

One selfish act to get her back, Marco! Is that so much to ask? His head voice hissed. Get her back!

Marco looked at the demon and sighed.

He shook the demon's hand which burst into blue flames. Marco didn't flinch though. The fire didn't burn him nor felt hot.

The whole world—no, all worlds—seemed to freeze.

"The deal is sealed…" The demon said. "Nice doing business with you… the deal will take effect when you wake up!"

"What do you mean?" Marco asked but he knows what's happening.

His eyes began to be heavy. His body is slowly surrendering to sleep. Marco felt his hand slip off the demon's hand. The place began to fade back to his room.

Marco watched as the demon fixed its bow tie and top hat.

Soon, Marco collapsed on the cold floor.

"Don't worry kid… you'll have your princess soon… and so is my share of the deal…" The demon began to open a portal but before leaving it gave Marco one last look and said: "By the way, the name's Bill: Bill Cipher."

With that, Marco's vision faded into black.

She will be with you tomorrow… but what's with the feeling that something evil has been unleashed?

That's the end of the Prologue.

Sure Marco acted selfish. Give the guy a break. He's sad and desperate to get the girl he loves. And Bill's words got him alright. Bill is the biggest conman in the whole multiverse, he can manipulate anyone.

Anyways, I hope you liked the prologue, next part will be soon! And it will be on the weird town of Gravity Falls…

…

Gravity Falls belongs to Alex Hirsch

Star vs the Forces of Evil belongs to Daron Nefcy