This fanfiction goes with a theory I've had for awhile now, it could actually happen in the show for starters, and I have evidence to prove this theory (and no, it's not just because the Holy Mackerel has a fish in it's name, there's more than that.) so enjoy, I guess? Also, I've seen a lot of people hate Mermando for only being a sub-plot character, being boring, and only used as Mabel's love interests. Well, for those of you who think that of Mermando...
Buckle up. Cause this is going to be one hell of a ride.

Gravity falls was always a secretive place, each day there was something you would find out that was kept secret for days, weeks, months, even years. Nothing was truly known about this town. And it wasn't like the townsfolk known anything, they were brainwashed each day for just hearing about a paranormal creature of some sorts rolling about this town. The only people who knew or suspected paranormal and secretive things in this town were the Pines family, that including their friends, Soos and Wendy. The Pines family wasn't the whole Pines family, it was just Stanford, now called "Ford", his twin brother, Stanley who is called "Stan", and their great-niece and great-nephew, who are also twins, Dipper and Mabel Pines. After asking Ford about the Society of the blind eye, Dipper started asking if there were other cults or secret organizations in Gravity falls. "There is one that really worries me," Ford said, tapping his fingers on the table. "It's called 'The Royal order of The Holy Mackerel'. Nobody knows what this group is actually doing, or if it's even here, but it's said to have very evil intentions and live somewhere around the Gulf of Mexico so no one in this town expects a thing. They seem to spy on the town by either bottles, or Bill Cipher."

"How long have they been here? And how did they get Bill Cipher to join them?" Dipper asked, Ford sighed in response. "That I cannot answer, but they are said to be there for centuries, they have a new 'leader' every hundred years. I've also heard that this 'leader' has to have executed someone who knows all the secrets of Gravity falls, if they fail, they would be executed." Dipper listened intensely until he heard the sound of his twin sister's voice. "Dipper! You're gonna miss the season premiere of Duck-tective!" Mabel shouted. "Get your big butt over here! You too, Grunkle Ford!"
"Ugh, well, can't miss Duck-tective." Dipper sighed, Ford gave out a chuckle, "I guess we can wait till tomorrow to talk about this. It's not really important anyway, the holy mackerel isn't even proven to exist anymore, and even if they did, I don't think they'd be after us."

Although, there was a different scene going on near the Gulf of Mexico. There was a secret underground building, inside that building, was the symbol for the holy mackerel. A man wearing a dark brown robe, covering his entire face, rolling the leader who looked very small and was in a mobile tank. The man continued walking until they reached a long table filled with other men in robes, covering their entire faces. The man then went to his seat in the table. "Thank you, mi amigos." a deep voice with a Spanish accent spoke as he lifted up his robe to reveal himself. He was tan-skinned, wore a dark green robe, and had long, wavy dark-brown hair. He also had small mustache hairs, a missing tooth, and a clamshell necklace. He also wore a black eyepatch on his left eye with a red triangular imprint on it.
"The holy mackerel, we have followed your orders as promised. The Queen of the manatees is now executed without even a trace of evidence that that was you who had done the crime." One of the men said, Mermando gave out a chuckle. "Excellent," He said, then he rolled himself over to three skeletons; a merman, mermaid, and a mer-infant. He placed his hand on the shoulder of the merman, "you hear that papa? Me and that so-called 'queen' didn't go well, in fact, it was so bad, she sure didn't deserve a divorce!" He laughed as he hugged the mermaid skeleton, "and mama, always criticizing me for what I can and cannot do, well, that's what you get mama! Not so pretty anymore, huh?" He then went over to the infant. "Oh and Finn, my little brother. I wouldn't have ended your life if it wasn't for your dear wrenched parents, they would've turned you, changed you. You would've became just like them. So I had no choice but to end your miserable life, mi amigo!" He then laughed menacingly. Then rolled back to his seat.

"And, my good friends, I have decided on executing the people in our dear little town of Gravity falls for knowing too much." Mermando lets out a chuckle. "Holy mackerel, are you sure you want to take on this quest yet? If you fail, you would have to be execute-" Mermando cut the man off by placing a blade under his chin. "Relax mi amigo, I have done a great job in killing my family and my ex-fiancée, no? And I escaped death many, many times, haven't I? Surely I could do a great job in this challenge." Mermando then places a sheet of paper on the table. "It's about time we sent another bottle message to my dear Mabel. After all she is related to the subject, or should I say, subjects I'm looking for." He cracked a smile. "She probably still thinks I'm already married to that manatee by now. Naïve girl. She and her brother of hers don't know who I really am." Mermando then smiles and laughs menacingly, knowing his plan is already in motion.

Butthurt comments about why Mermando isn't apart of the holy mackerel and why he isn't apart of anything important of Gravity falls once so ever, in 3, 2, 1...