Twilight and her friends swam down through the ocean, swimming past coral reefs and columns of bubbles as they followed Mermaidyn. They saw many different creatures that most of them had never seen before or heard of in their lives. And while they did stop and take in each different animal, it didn't last for very long each time.

"This is quite simply divine." Rarity took in each and every one of the sights. "Who knew that the ocean held so many wonderful secrets?"

"Not only is the underwater world quite lovely, but it also houses many different aquatic Yo-kai like myself." Mermaidyn raised her finger as she skidded to a halt. "Speaking of which, there's a school of fish Yo-kai incoming!"

A whole school of yellow fish swam past the Main Six. Some of which had plain blue stripes and a blue X shape on their forehead, and others with red manes and flippers. The plain yellow ones had tails made out of paper fans while the ones with red hair had sharp knives coiled up in their tails.

"Ooh! What are those?" Fluttershy jabbed a finger towards the fish.

"Those are Yo-kai. More specifically, they're Fishpicable and Rageon." Mermaidyn swam over to Fluttershy and pointed them out. "The simple ones are Fishpicables. If you get inspirited by them, then you hate everyone and everything with a burning passion."

"Hence the name, right?" Twilight asked.

"Exactly." Mermaidyn nodded. "As for the other fish, those are all Rageon. Much like Fishpicable, Rageon hates everything—including himself. He constantly flails his knife around and is very hard to befriend these days." She then swam away from Fluttershy and continued into a more open part of the ocean. "There's more sea Yo-kai that you'll meet as we swim down, but we'll only have time for brief explanations. We need to get to my kingdom as fast as possible!"

"Kingdom?" Rainbow Dash cocked a brow at that and followed after Mermaidyn once more.

"Ooh! Ooh! We're actually going to a super-secret mermaid kingdom thousands of leagues under the sea?!" Pinkie Pie beamed from ear to ear as she followed the rest of the group.

"Yes. One that's come under fire recently from forces beyond our comprehension." Mermaidyn looked back at the girls as she came past a large group of shark-like creatures with arms and legs. "Whoa! Swim back, girls!" She threw out her arms and came to a grinding halt once again.

"More Yo-kai?" Applejack took a look at the sharks' arms and legs, as well as the geta on their feet. "Ah'm guessing that they don't look like particularly friendly folk either."

"Well, they're not entirely hostile, but they're still brutes nonetheless." Mermaidyn cracked her knuckles in preparation for combat. "The blue ones are called Chummer. They're Yo-kai that make you loiter and loaf around after school. And then, they eat you alive."

"Seriously?!" The Main Six jumped at Mermaidyn's description.

"Yep." Mermaidyn lowered her brows and launched her finger towards the green sharks. "As for the other sharks, they're called Shrook. These ones don't eat people alive, much like their Chummer counterparts. Instead, if Shrook inspirits you, then you become really terrible at something you're good at." She looked back at the girls. "So any special skills or talents that you might have, Shrook will make you completely pathetic at them all."

"He's basically the ultimate counter to our Cutie Marks..." Twilight could only gaze in the shark Yo-kai in awe as they swam directly over her and towards another part of the ocean. Once they had left, her attention returned to Mermaidyn. "Anyway, let's move on. If your kingdom's in trouble, then we'd better save it."

"Speaking of which, what was the problem again?" Rainbow Dash tipped her hand.

"Fish Yo-kai and mermaid Yo-kai used to be at peace. We used to play with each other, help gather food for one another, and just generally be happy together." Mermaidyn gave a smile that quickly melted away as she swam on. "But then one day, this pack of horrible fish came and attacked our kingdom. They ravaged our kingdom, destroyed our homes, and left Mermadonna and Mermother near the brink of death with their horrible attacks."

"Oh my..." Fluttershy winced as some of her hair draped around her face. "I don't like this story much."

"Mermaidyn. Tell me." Twilight swam up to her. "Did these fish have anything special about them? Like a different colour or pattern?"

"Well, now that you mention it, they were all purple and black, with malicious red eyes." Mermaidyn gently raised one of her fingers. "They groaned and moaned about how everything would die and that we would 'bow before his wrath'..."

"Purple and black? Red eyes? Bow before his wrath?" Pinkie Pie used her fingers to list off each of Mermaidyn's points. Once she had put them all together, she let out a large gasp. "We're dealing with none other than the evil King Squidra! Sombra's evil squid brother that was banished to the arctic sea, never to be seen ever again!"

"Pinkie. No. Just... No." Twilight face-palmed and shook her head. "Black and purple skins, evil red eyes, and the will to hurt and destroy can only mean one thing." She tightened her hands into fists.

"Dark Yo-kai!" The other girls chorused and darted their eyes around, their arms lightly quivering as they imagined the thought of King Void's army destroying Mermaidyn's home.

"Yes. Dark Yo-kai." Twilight nodded. "King Void, Kin and Gin, or one of the Dark Ones was here, turning your fish into Dark Yo-kai in the hope that they would cause chaos and disrupt the peace of your world."

"I've heard tales of this King Void character, but I never thought that his tyranny would spread to the ocean floor as well." Mermaidyn glanced back at the others as they went farther and farther into the water. "But what about the Dark Yo-kai? Are they similar to Dame Dedtime's Wicked Yo-kai?"

"No. They're much, MUCH worse." Twilight sighed and put her hand on Mermaidyn's shoulder. "Unlike Wicked Yo-kai, who inspirit humans to cause unruly chaos, Dark Yo-kai care only about murder, pain, and destruction. No matter whether it's a human, Yo-kai, or pony, a Dark Yo-kai's only goal is to see them bleed and or die. They don't care about anything else other than serving King Void. It's like they're all mindless zombies..."

"They do sound pretty bad." Mermaidyn's brows elevated as a glimmering light shone in the distance. "Actually, hold that thought. We're here! The Mermaid Kingdom!"

The girls all looked forward and saw a rather well-kept kingdom, with large structures made out of dirt and earth, with seashells lining the walls and ceilings. There were long and winding roads with lots more Mermaidyns swimming through them, with smaller, non-Yo-kai fish swimming alongside them. The entire town as a whole was rather expansive, with a long and winding road leading up to a castle on the nearby cliffside.

The castle was made out of golden sand, with many more decorations embedded into the walls and towers. There were large spires pointing towards the shimmering roof of the ocean, with many different doors and windows to be seen on the sides. The castle door itself was made entirely out of wood, with golden knockers attached to each door.

"So, this is your kingdom?" Twilight swam with Mermaidyn towards the front door of the castle with the rest of her friends close behind her. "It looks a lot different than I was imagining, I have to say."

"It's your first time under the water, I take it?" Mermaidyn chuckled and covered her mouth with her hand. "It's alright. Not many land creatures get to see the world beneath the waves, so I think that you're all really lucky to see this place, let alone get a guided tour of the many different sea Yo-kai."

"Again, if only I had my sketchbook." Rarity's jaw hung wide open as she swam past all of the different underwater architecture and marine life. "I'm getting so inspired simply by looking at this stuff!"

"We were only able to completely rebuild this place yesterday." Mermaidyn stretched her arms out towards the buildings. "King Void's Dark Yo-kai did so much damage in such a short amount of time that it's a miracle that we were able to put it all back together. And that's not even counting the other dangers that come to this kingdom."

"Other dangers?" Pinkie Pie tilted her head. "What kind of meanie-pants would ever want to destroy such a pretty kingdom like this? It's just a crime against the fish/mermaid species!"

"I know. But then again, beauty is so fragile." Mermaidyn swam towards the castle doors and opened them wide. "All it takes is one person to turn something pretty into an absolute mess."

"Hear hear." Rarity nodded. "Those ruffians just wouldn't understand beauty if it slapped them across the face with a satin and sapphire-encrusted glove."

As the girls ventured into the castle, the same Yo-kai that had jumped into the ocean from before pulled its' head from the nearby rock formation. Instead of its' trousers and shoes, it now had a purple mermaid tail with black fins. It slowly took the mask off, revealing a face silver moustache and a monocle on the right eye.

"At long last. The Mermaid Kingdom is within my grasp." He coiled his fingers around the rocks and cracked a malicious grin. "After eons of waiting, these Yo-kai shall truly understand and appreciate my masterpiece." He raised his conductor's baton and flicked and swished it around, causing purple steam to appear behind him.

Inside the castle, the girls swam over a red-rug with golden edges and tassels as they passed by multiple pictures and vases. They saw more Mermaidyns patrolling the corridors, but they were carrying small spears in their arms and were coated in silver armour. The new Yo-kai simply passed by Twilight and her friends, paying them absolutely no mind whatsoever.

"We're almost at the grand chamber, girls." Mermaidyn picked up the pace and bolted towards a giant golden door with jewels of all different colours both on the frame and on the actual door itself. "Through these doors is the highest authority in this fair land: Queen Mermother and Princess Mermadonna."

"Mermother and Mermadonna?" Twilight parroted.

"They're both mermaids, much like myself if you couldn't guess. But they're much more beautiful and alluring than the rest of us down here." Mermaidyn smiled and opened the doors. "Now, I want you all to be on your absolute best behaviour. Princess Mermadonna gets really whiny when something goes wrong."

"Enter!" A voice boomed from the other side of the doors.

The girls all went through the doors and came up to a large hall with spiral pillars surrounding them all. The doors slammed shut behind them as they came towards a large throne that pointed out from the bottom of the floor. The open gaps in the pillars led out to the rest of the ocean, with nothing but blank water to be seen for miles and miles.

In the middle of the hall, there was a large throne shaped like a clam. Sitting on the throne was a mermaid with sky blue hair and black branch-like horns poking out of them, fins for hands, a blue tail with silver fins. She wore a red veil around her wrists that was much similar to that of the one Mermaidyn was wearing. Next to her was another mermaid who looked exactly the same as she did, only with black hair, a white veil, a golden tail and fins, blue eyes, and blue branch-like horns.

"Mermother! Who in the briny deep are these mermaids?" The golden-finned mermaid whined and pointed to Twilight and her friends.

"Oh, pipe down, Mermadonna." Mermother raised her hand towards the mermaid. "We're in a time of crisis as it is. Don't you go off on one of your teenage episodes now." She turned towards the group. "Now tell me. Who are you, and what can you possibly do for us?"

"Twilight Sparkle. Princess of Friendship." She swam forward slightly. "These are my friends. Pinkie Pie, Applejack, Rainbow Dash, Rarity, and Fluttershy. We came to this kingdom because Mermaidyn here asked for help."

"How interesting." Mermother swam up to Twilight and examined her body, looking especially at her hair, fins, and horn. "You do look much more exotic than a regular mermaid," she said. "Tell me. What ocean are you all from?"

"Oh, we're not from any ocean. We're from the surface world." Twilight pointed up to the ceiling.

"Surface dwellers?" Mermother's eyes shot open at that remark. She quickly cleared her throat and took a deep breath. "Ahem. Well, if any case, that would explain why you're wearing the Yo-kai Watch. But it doesn't really explain why you look all look like mermaids."

"They're probably more of those horrible black Yo-kai." Mermadonna reviled at the sight of the girls. "Get them out of here before they do us more damage!"

"Mermadonna! Hold your tongue!" Mermother glared at Mermadonna before turning back to Twilight and her friends. "Excuse her. She's not exactly out of what those humans call 'puberty' yet."

"It's perfectly fine." Twilight nodded. "Now, I believe that King Void and his Dark Yo-kai were what destroyed your kingdom. We've had a fight with them before back in Ponyville and we'd be glad to help you out in defending this place if they ever come back."

"Well, that's all well and good, but I don't think that there's really much you can do if all of your Yo-kai friends are land-based." Mermother shook her head. "But perhaps, I can persuade Mermadonna to lend you a hand if the Dark Yo-kai strike again."

"Mother, do you know how much those freakish Dark Yo-kai ruined my hair and make-up?" She pouted and scoffed like a teenage girl would. "I've had to cancel multiple concerts and do lots of charity work because of those stupid things and now you expect me to fight them?! No thank you!"

"Be grateful that you're even in this kingdom at all, Mermadonna!" Mermother swam up to her face and pressed it against her own. "You've been such a spoilt brat for the past few years, so do not try your patience with me or our fine guests! You're going to fight the Dark Yo-kai and you're going to like it one way or another, missy!"

"Ugh! You never pay attention to anything that I want!" Mermadonna screamed and swam out of the hall and up to the top of the castle.

"What a repulsive little toddler. Whining about some make-up rather than her destroyed kingdom." Rarity tossed back her hair. "Honestly, darlings, there's a time and a place for everything, and I would say that mermaid make-up isn't really that important."

"Spoken like a true lady." Mermother swam towards Rarity and smiled. "I might just have to give you a Yo-kai Medal of my own once this whole thing blows over. But for now, we have more pressing manners to attend to." As Mermother returned to her throne, she didn't even notice the giant shadow rising from the open waters behind her. "First of all, we shall have to rally the troops and train them hard should these Dark Yo-kai ever come back an—"

"Mermother, look out!" Twilight lit up her horn and fired a spell at the shadowy monster just as it lunged a claw towards the throne room. The bolt simply went through the monster and into the waters beyond. What?! It's a Yo-kai?!

There was a loud crash as the claw slammed against the hall, destroying the pillars and gold door. Rubble toppled all around Twilight and her friends as the hand retracted back towards the giant monster, with clouds of dust rising from all of the falling debris.

Everyone coughed and fanned the dust away as they recuperated from the attack, with no-one looking like they'd been majorly harmed by the monster out in the ocean.

"What in the heck is that?!" Rainbow Dash backed up against what remained of the wall as the giant shadow beast glared down at the group.

"Well, whatever it is, it's not friendly..." Twilight saw giant blue eyes shine down on the group like spotlights. It let out a massive roar that echoed across the ocean as the monster began to take on a visible appearance.

The beast was much like Mermaidyn in every aspect, only now, the hands were now like massive claws, there was a malicious and warped face with an evil grin and long, pointy teeth, and the veil around her wrists was so enormous that it looked like it could wrap up the castle and drag it off of the edge of the cliff.

"People of this kingdom! All shall bow down before me!" The beast thundered in a distorted feminine voice. "I am the tyrant queen of the ocean. The demonic deity of disaster down in the water! I am... MERMONSTER!"

Author's Notes:

If you guys and gals are wondering about Mermonster at the end of this chapter, then think of her like a Yo-kai Watch Busters version of Mermaidyn. Seeing as how Rubeus J and Hardy Hound are different versions of Jibanyan and Komasan, I figured that more Yo-kai could get the Busters treatment. If you like this idea, then be sure to let me know what other Yo-kai could get the busters treatment and join the ranks of Rubeus J, Hardy Hound, G Baban, Prison Breaker, and the Whismellow Man.

But for now, just have a good day and take it easy.