That Yellow Triangle Guy - The Owl House X Gravity Falls

The Summoning

Luz, King and Eda were all in a dark, seemingly hidden room. A ritual was laid out on the floor, a circle of candles with a framed picture of Amity in the middle. Luz had many emotions flowing through her, most of them being related to either fear or reluctance, but the only thing she could legibly communicate when she turned to look at Eda was a sullen "Why...?"

Eda looked at her like it was any other day. "Oh, this? This is just some empty space left over from construction that I discovered back when I went to Hexside. Trust me, kid, a demonic ritual isn't even in the top five weirdest things that's happened in this place." She laughed to herself.

Luz looked at Eda confused, yet still very tense. "What? No! No, I-I was referring to the framed photo of Amity. Why do you even have that...?"

Eda shrugged. "I broke into the Blight family's house once and stole these pictures as leverage. Most of them were just boring family photos showing their 'happiness'. Bleh. Luckily, I forgot to throw some of them out." Luz was not eased at all by this explanation.

"Oh well." Eda said. She reached into her hair and handed a piece of paper to Luz. "Now, just read the incantation." 

Luz had to double take. "Wh-wait, what!? Wait, why do I have to do it?!"

Eda looked at Luz with a mild frown. "Would you rather I summoned it and I told the demon what to do?"

Luz silently relented, and started reading the incantation. "Triangulum... entangulum. Vene forest- foris, dominus omnium... Vene foris videntus omnium?"

Luz tensed her entire body and prepared for impact, but nothing came. She opened one of her eyes. Nothing. She was very confused.

Eda looked over her shoulder. "Maybe you didn't read it fast enough?"

Luz sighed. "Dios mío..." She cleared her throat and started reading again.

"Triangulum, entangulum. Vene foris dominus mentium. Vene foris videntis omnium!"

She tensed up yet again, but nothing seemed to happen. She looked at the framed picture of Amity, waiting for something to happen.

King went on his tippy-toes and put his hand on Luz's shoulder to try and comfort her. "It's okay... Maybe Eda just wrote the incantation wrong?" Eda put her hands on her hips. "Hey!"

Luz looked at Eda, still kind of processing the whole 'trying to use a demonic ritual' thing. "Maybe I'm just not pronouncing it right? Or maybe... ugh..." Luz clasped her stomach and fell on her knees.

"LUZ!" King and Eda rushed to her side in worry. Luz made a guttural sound before her eyes glowed blue and she started involuntarily chanting, the words more intense the more they make it out. "Egassem sdrawkcab. Egassem sdrawkcab. Egassem sdrawkcab. Egassem sdrawkcab! Egassem sdrawkcab!"

A pitch black triangle with one lone eye rose up from behind the framed picture, laughing maniacally. It grew arms, legs, a top hat and the triangle itself started to turn yellow as all of the surrounded started to turn monochromatic.

"Oh, oh, the Boiling Isles, it is good to be back!" The triangle rose closer and stared Luz directly in the eyes. "The name's Bill Cipher! Who are you three supposed to be, some kind of ragtag gang of rejected marionettes? Haha! Just kidding, I know who all of you are."

Luz took a few steps back, in awe of this... thing. Eda and King get in front of Luz and get into an attack stance. Eda points her palisman at Bill. "You lay one finger on her, and I'll-"

"Boring." Bill snaps his fingers and Eda and King freeze in place, turned as gray as their surroundings. Luz gasps and points at Bill accusatively. "Wh-what did you do to them!?"

"I froze them! Anyway, what do you need me to do for you, Luz?" Bill asked. Luz recoiled at the sound of her name. "H-how did you...?" Bill happily kept going. "I told you, I know who you are! I know lots of things, in fact!"

"LOTS OF THINGS."

Luz took a bit of time to process what just happened before trying to ask Bill to help her. "Well, uh, Mr. Cipher-" Bill interjected: "Bill is fine! Or William, if you have the grammatical prowess of a U.S congressman. My friends call me Billy C!" Luz looked at him incredulously. "...Really?"

Bill laughed maniacally. "No."

Despite how unhinged this triangular maniac seemed to be, Luz couldn't help but be oddly comforted by the "joke" he made. Perhaps she sees him as a bit of a kindred spirit. A 'fellow weirdo', as she might put it. Whatever the case, Luz no longer saw Bill as a threat, but rather a potential ally.

Luz lightly smirked. "Well, Bill, I was wondering if you could help me against this girl who's kind of my rival. I may have accidentally created an everlasting oath with her, and-" Bill interrupted her, his hat hovering over the tip of his head in what appeared to be shock. "Whoa, whoa! An everlasting oath! Those things are everlasting!"

Luz looked at him with slight irritation. "Yeah. I know." She sighed and tried to be amicable in her next sentence. "I was just wondering if you could help me win against her in this duel we have to do?"

Bill looked at her, with excitement. "What's this enemy's name, anyhow? Are they some kind of half-goat half-ox demi-oxymoronic child murderer cult leader?"

Luz shook her head. "No, she's just a really mean girl. She's not a criminal."

"...At least, as far as I know." Luz stared into space for a little before zoning back in. "Uh, and her name is Amity Blight!" Bill laughed to himself before turning away.

"Amity Blight..." Bill said to himself, almost as if he was checking a database located entirely within his mind. His eye widened and he turned back to Luz. "You know what, kid? You've convinced me! I'm sold! I'll help you with this 'Amity' problem you have, and in doing so you'll owe me a favor later. Deal?" He reached his arm out, a blue fire engulfing it.

Luz looked amazed before hesitating slightly. "That's not real fire, is it?" Bill looked at her and said "Is fire blue?" Luz thought to herself and said "Sometimes."

They stared at each other for an uncomfortable amount of time before Luz shook his hand. "So, what do you need me to do?" Bill asked. "Want me to go into her mind and trigger all of her traumatic experiences so she calls off the bet?"

Luz looked mildly uncomfortable. "Uh, that may be a bit overki- Wait, you can go into people's minds?" 

Bill nodded his eye. "Sure I can! It's one of my main hobbies aside from organ failure and playing the otamatone!"

Luz looked intrigued yet also somewhat disturbed. "How is organ failure a hobby?" Bill did not react to this in any way whatsoever.

"Anyway," he said, nonchalantly, "what do you want me to do to this 'Amity' gal?"

"I don't want you to do anything to her! I just..." Luz scratched the back of her head. "...want you to help me win the duel against her." 

Bill immediately got what Luz was saying. "Ohhhh, you want to cheat! I can help you with that."

"No no no no no!" Luz shook her hands and took a step back. "It's not cheating! It's not cheating. It's just... a little external help. You know, to make things equal. I'm sure Principal Bump would allow it if he knew. Yep. Not cheating."

Bill Cipher was not convinced in the slightest. "Boy, you keep telling yourself these lies and you'll end up dead in a prison cell!" He once again laughed maniacally.

Luz was slightly weirded out. "Okay... well, I guess I'll see you at the duel, then?"

Bill gave her a thumbs up... with his eye. "Yep!" His eye slunk back into its socket. "I'll be there faster than you can say 'Catchphrase for the end of chapter'!"

Luz thought to herself. "Catchphrase for the end of chapter? What does that even mea-"