Mermonster let out an ear-piercing screech as she glared down at Mermother and her guests. Flexing her claws, she launched spikes out of her tail that barrelled through the water and detonated on contact with what remained of the walls, completely decimating the structure with little effort.

"Mermaidyns! To battle!" Mermother swam down to the back of her throne and pulled out a golden trident, twirling it around in her arms. She pointed it towards Mermonster and shot a bolt of yellow energy at the giant Yo-kai. "Don't let this fiendish Yo-kai destroy our world!"

"Why the heck is that thing so huge?!" Rainbow Dash pointed at Mermonster, who began to battle with the hundreds of Mermaidyn soldiers.

"There are some Yo-kai that are much more vicious versions of their regular-sized counterparts." Mermother looked back at Twilight and her friends. "We call these Yo-kai SS Rank Yo-kai. They're incredibly dangerous and destructive, often wreaking havoc on unsuspecting people to exercise their power."

"So, it's not a Boss Yo-kai?" Twilight lifted a brow as Mermonster began swatting at the Mermaidyns.

"Well, technically, yes and no. SS Rank Yo-kai are harder to befriend than a Terror Time Oni." Mermother shot a blast of blue energy at Mermonster's face, causing it to reel slightly. "They can be defeated, but it takes a massive group effort to do so. Think of them as a giant raid boss from a multiplayer game."

"DIE!" Mermonster screeched and swung her claws, sending the Mermaidyns all crashing into the castle. "Chaotic Waters!" She clapped her hands, sending a massive current towards the mermaids. She ripped the water apart with her bare hands and then smashed it back together, slamming the mermaids into one another.

"Whoa! This thing is crazy!" Twilight was jerked around throughout Mermonster's attacks.

"Usually, we leave these SS Rank Yo-kai to the Yo-kai Watch Busters team, but since it's deep under the sea, we're the ones who have to take her down."

"Yo-kai Watch Busters?" Applejack raised a brow as she recovered from the last attack.

"Best not to talk about that for now," said the Mermaidyn that had escorted them to the kingdom. "For now, we need to defeat Mermonster. Anything that you pony-mermaids can do, do it now. And Twilight, if you can summon any water-based Yo-kai, then that'd be great too."

"I don't have any fish or water-like Yo-kai unfortunately." Twilight shook her head. "And besides, my Yo-kai Medallium's back on the cruise ship with Whisper and Jibanyan."

"Then you'll just have to do what you can to distract it." Mermother brandished her trident and pointed it towards Mermonster. "Just do whatever you can to attract its' attention. Slap it in the eyes with your tail, insult it, anything you can do to make it focus on you and you alone will help."

"Distract? We don't do distractions, Mermother." Rainbow Dash swam up towards Mermonster and cracked her knuckles. "This big Yo-kai needs to learn some lessons and leave us all alone!" She sped right towards the giant Yo-kai and punched her in the eye.

Mermonster screamed and covered the eye that got punched, reeling backwards as she recovered from the pain. She clutched her eyes so much that she unintentionally scratched and cut the flesh around them. She pulled her claws out from her face and glared at Rainbow Dash, causing giant red lasers to shoot out of her eyes.

"Yikes!" Rainbow dodged the lasers and swam down through the drop-off, sliding through the red beams of light by the skin of her teeth. "Whatever this thing is, it's probably the most dangerous thing I've ever had to fight!"

"Not only does it look like a monster, it looks like one of the most beautiful monsters I think I've ever seen." Rarity looked at how Mermonster's scales glistened in the light and how luscious her hair flowed as she chased after Rainbow Dash. "Shame she's a hostile Yo-kai. If she weren't, I would be asking to be her friend in a heartbeat."

"Never thought you'd be complimenting a beast like that, Rarity." Applejack crossed her arms and swam after Rainbow Dash.

"What can I say? Even some creatures like Mermonster can look beautiful while acting brutish." Rarity glanced at Applejack as she and the rest of the Main Six swam into the battle.

"Mermaidyns! Press the attack while our guests distract Mermonster!" Mermother lunged her spear at the giant Yo-kai, sending her army charging towards it. "As long as her attention's on Mermonster, she won't be focusing on you. Unload everything that you have on her! Soultimates, inspirits, the works!" She swam out and looked at the tower that Mermadonna had hidden herself away in. "But we can't win without your help. We need everyone's help to defend the kingdom..."

Mermonster snarled and rushed after Twilight Sparkle, who was now using spells of all different varieties on Mermonster. She fired magical bolts and blasted the rocks around her to try and get them to fall on top of the massive Yo-kai pursuing her, but they either bounced straight off of her large body, or were smashed into pieces with a swing of her hands.

"Worthless little worms! Give up and die already!" Mermonster snarled and pointed the tips of her claws at the girls, sending streams of lightning through the water.

"Girls! Split up!" Twilight shouted as the lightning approached her and her friends.

The Main Six broke off into different directions as Mermonster's lightning scraped by their bodies. They all swam into holes in the wall as the giant Yo-kai stopped in front of it.

"Where are you hiding, little mouseys?" Mermonster's claws dug into the cliff wall as her eyes gazed into the holes. The light she emanated off of her glares lit up the caves, but she couldn't find Twilight or her friends anywhere inside. "Don't make me do this the hard way. I would REALLY hate to get nasty on you non-Yo-kai interlopers..."

As Mermonster looked at the caves, hundreds of Mermaidyns attacked the giant Yo-kai, striking them with their tails or punching her across the face. They cast inspiriting flames on the titanic Yo-kai, but none of them connected as they slid right off of her back and down her scale-covered tail.

"Alright, girls." Twilight lit up her horn and brought all of her friends into a group huddle as Mermonster's attention shifted to the Yo-kai. "As long as we stay in here, Mermonster can't possibly get us."

"So, what's the plan, Twilight?" Rainbow Dash asked as the girls all put their arms around each others' shoulders.

"Mermonster's an SS Rank, but she can be brought down by a massive co-ordinated attack." Twilight looked out at the battling Yo-kai. "While we distract her with our spells, taunts, and games of hide-and-seek, we can cause her to stop fighting the Mermaidyns, leaving them a clear shot to do some massive damage."

"Got it." Pinkie Pie nodded and darted off to one of the cave entrances, looking out at Mermaidyn. She reached into her hair and pulled out a flashlight, shining it into Mermonster's eyes. "Hey, ugly! Come and play!" Pinkie wildly shook the light around, agitating the Yo-kai's eyes and causing her to reel.

"You again?!" Mermonster charged at the hole like a colossal torpedo and rammed into it, trying to slam into Pinkie Pie. "Just give up!"

"Oopsy-daisy! Over here!" Pinkie Pie pulled the corners of her mouth and stuck her tongue out at Mermonster. "Gotta catch me first!"

"Terrorpedos!" Mermonster fired hundreds of spike-tipped missile-shaped bulbsthat travelled through the sea and left behind bubble streams in their wake. They were travelling directly towards Pinkie, who swam back in to avoid the series of explosions that followed. "Destroy these mortal fools!"

"Nope!" Pinkie Pie popped out of another hole and swam away from another of the spiked warheads and went through another hole. "You're really bad at this game, aren't you?" She narrowly avoided a slash from Mermonster's claws as she went into another part of the cliff altogether. "Missed me! Missed me! Now you've gotta kiss me!"

"ENOUGH!" Mermonster barrelled her claws into the hole and ripped a large chunk of the rocks out, leaving Pinkie Pie exposed. "JUST SHUT UP AND DIE, MORTAL!" She roared and was about to strike Pinkie with another attack, raising her hand high above her head.

"No, you don't!" Rainbow Dash swam in front of Mermonster's face and slapped both of the Yo-kai's eyes with her tail. "Rainbow Dash! Fastest flier in Equestria, now the fastest mermaid in the sea! Catch me if you can!" She put her hands against her head and blew a raspberry, swimming away from Mermonster.

"GRAAGH!" Mermonster slapped Pinkie Pie away with her tail and shot herself towards Rainbow Dash. "Do you think you're going to get away from me like that, you little twerp?!"

Twilight chased after Mermonster with Mermaidyns and her other friends close behind her. They watched as Rainbow Dash swooped, dived, and dashed all around Mermonster, making her spin around in a frenzy as she tried to keep up with her. Once the SS rank Yo-kai has swirls in its' eyes, Rainbow joined her friends as the Mermaidyns swam forward and dealt their attacks.

"She's reeling! Now we've just gotta bring this big feller down!" Applejack swam onto Mermonster's neck and grabbed on tight with her arms, causing the giant Yo-kai to thrash and flail around trying to throw her off, while also dealing with the other Yo-kai. "YEEEEEE-HAW! This is the biggest thing I ever done rodeo'd!"

"GET OFF! GET THE HELL OFF OF ME!" Mermonster cried out as she barrell-rolled and somersaulted in an attempt to shake Applejack off of her. She tried to grab the back of her neck to pull the country girl off of her, but every time she reached over to her, the Mermaidyns hammered down on her with physical and elemental attacks.

"She's still standing. We need more Yo-kai!" Twilight looked back at the castle which only barely stood after the initial attack. "Let's just hope Mermother can convince Mermadonna to come to her senses and help out." She looked back at Mermonster who swam towards the top of the ocean. "And fast! Who knows where we are in this ocean?"

"Fine then. We'll take this battle to the surface!" Mermonster flew towards the roof of the sea, managing to throw Applejack off of her neck at long last. "If we're going to end this, then it'll be in a place where you Mermaidyns can't do anything!"

"Looks like it's game point to Jibanyan's team. Service!" Whisper was up on the cruise ship, playing a game of volleyball with Jibanyan and a bunch of other Yo-kai. He threw the ball high and smashed it down on Jibanyan, who knocked the ball back into the air, flip kicked, and smashed it into Whisper's body.

"Nyeah! We win!" Jibanyan landed and hugged the rest of his team, running over to Whisper. "Your team wasn't so bad either, Whisper. Good game."

"Yes. Well, thank you for suggesting the idea, Jibanyan." Whisper got up and shook Jibanyan's paw. "Though I do still wonder where Twilight and her friends went..."

"Hey, what's that?!"

Whisper and Jibanyan followed a crowd of ponies that were all gathered on the front of the cruise ship as they saw bubbles gathering in the water ahead of them. They appeared and disappeared in rapid succession, popping one after another. It was only a small area of bubbles, but it soon got bigger and bigger as a dark shadow appeared around them.

Soon, Mermonster burst out of the bubbles, deluging the entire cruise ship and soaking every pony and Yo-kai on board. She roared as loudly as she could as she looked down on the ponies in front of her. She raised her arms, causing the water behind her to rise into two giant tsunamis. With a clap of her hands, she smashed the waves together, sending water everywhere around her.

"WHAT IN THE HECK IS THAT THING?!" Jibanyan jabbed a waterlogged paw at the monstrous Yo-kai.

"I don't know, Jibanyan." Whisper heard Mermonster scream out once again. "But judging from its' size and scale alone, it must be a Boss Yo-kai!" He pulled out the Yo-kai Medallium from his chest. "It's a good thing I secretly packed this thing in case something like this happened. Now all we need to do is find Twilight and her friends." And with that, he flew off towards the front of the ship.

Twilight and her friends resurfaced and took big gasps of air as they saw Mermonster towering over the six of them. They watched the rest of the Mermaidyn army appear as well, bobbing their heads out of the water as they gazed upon the giant Yo-kai's malicious visage.

"Twilight!" Whisper spotted Twilight immediately and flew down to her side. "Where in the world did you run off to? I was worried sick!"

"Whisper. Thank Celestia you're here." Twilight took the Yo-kai Medallium and opened it up, pulling out six Yo-kai Medals. "You're just in time, too. This SS Rank Yo-kai's been a real problem."

"Well, it's good to know that you're all—" he stopped and looked back at Mermonster "—wait. SS Rank? That's a phrase I haven't heard in a long time."

"Do you know anything more about these SS Rank Yo-kai?" Twilight asked Whisper as she activated the EX function on her Yo-kai Watch, giving it the five extra medal slots while re-altering the clock to show the computer screen.

"SS Rank Yo-kai are bigger, badder, and much more destructive versions of regular Yo-kai." Jibanyan hung himself over the rails and looked down at Twilight. "This massive one you're seeing is like another Mermaidyn, but just made extra-huge and deadly."

"Other SS Rank Yo-kai include—but are not limited to, Rubeus J, Hardy Hound, Prison Breaker, G Baban, Robonyan Mk. 28, and—"

"Alright, Whisper. I got this." Twilight took out her six medals and was about to slam her Yo-kai Medals into the watch.

"That won't be necessary, Twilight. In the language of humans, this Mermonster just played herself."

Mermother and Mermadonna burst out of the water and up into the air, twirling their golden and silver tridents at the giant SS Rank Yo-kai.

"Mermaidyns! Combine all of your Soultimates!" Mermother called out as she and Mermadonna took in purple energy. "Hit Mermonster with all of our waves and she'll be down for the count once and for all!"

The other Yo-kai took in energy for their own Soultimates, turning the sea around them into a dark shade of purple. The water ripped and shook around them as they closed their eyes and put their hands together. The activity around them shook and wobbled the cruise ship, throwing everyone on it around like peas in a frying pan.

"I will not allow that!" Mermonster slithered towards the army of Yo-kai, gnashing her teeth at them.

"NOW!" Mermother jabbed outward as the Yo-kai finished gathering energy.

"COLOSSAL TSUNAMI!" All of the mermaids called out together as an enormous wave rose up around them and consumed Mermonster whole. They flapped their tails and thrust their arms out, shooting the Yo-kai across the sea, far away from them and the ponies on the ship.

"No! NO! NOOOOOOO!" Mermonster screamed as she was catapulted across the water and shot halfway across the world, where she was impaled on a mountain and disappeared into an explosion of purple smoke.

Cheers rose up from the mermaids as Mermother and Mermadonna landed back into the water with a splash. They poked their heads up and looked out at all of their subjects. They waved out to all of them as they soon swam up to the two of them and hugged them gently.

"We did it!" Twilight swam up to the mermaid Yo-kai and threw her arms up. "We actually did it! We beat an SS Rank Yo-kai!"

"This calls for a celebration!" Pinkie Pie jumped out of the water and landed right beside Twilight and the Yo-kai. "We've saved the mermaid kingdom from Mermonster!"

"Congratulations, Twilight!" Whisper flew down to her and shook her hand vigorously. "I couldn't be any more proud of you for defeating such a tyrannical Yo-kai like that!"

"Nyeah! You're the best!" Jibanyan ran back up to the edge of the ship and waved to Twilight. "I thought that she had you for sure.

"Yes, everyone. Congratulations are in order. Unfortunately for you, they'll be for King Void, and not for your feeble kind."

The cheers were cut short as a massive maelstrom appeared in front of everyone, slowly pulling everything in the water towards it—including Twilight, her friends, the mermaids, and the cruise ship. The sky turned from a clear and sunny one into a sky filled with black clouds, lightning, and heavy rain.

At the center of the maelstrom, the Yo-kai that had been pursuing Twilight and her friends now stood proudly on a massive steel platform. The fins that it once had were now regular legs with trousers and shoes again, and the conductor's stick as the Yo-kai pulled off the golden face mask.

"How sweet. You make friends with such a lovely species, only for them to be lost forever to the very ocean that they tried to protect," the Yo-kai beneath the mask was revealed to have a female voice, with blue and black hair, purple eyes, and a devilish grin on her face. "Pretty ironic, don't you think? If our king had a sense of humour, he would be laughing and wheezing like a little kid at the irony of it all."

"Who are you?!" Twilight splashed water away from her as she looked out at the new Yo-kai.

"My name is Maelstro. Conqueror of the oceans and the Queen of Music." She swung her conductor's baton, causing a massive orchestra to join her on the platform, all of which fired musical notes that scratched Twilight and her friends' arms and made them bleed. "King Void has sent me to end your little charade and unearth Midnight Sparkle, so you won't ever be leaving this ocean alive. I'll make sure of it!"

Author's Notes:

I have a Patreon account!

If you beautiful fans want to donate to me and support my growing endeavours to write, then just click on the picture and choose how much you want to donate. You'll get special benefits, behind the scenes looks at my work and how I felt doing some of my other stories, and many more! And all for just chipping in as little as $1 a month.

Have a great day, be sure to click on the picture if you want to check out my Patreon and donate to it, and as always, I'll talk to you all later!