Before we start, here is a bit of backstory. There are 6 realms. The realm of vampires, pixies, humans, elves, dragons and werewolves. The mermaids and sirens live in the seas and are the only ones able to travel through the realms. Alas, their curse is that they have to remain in the sea at all times. If they go on land, they can't breathe.

Adrien Agreste is the vampire son of the oldest family in their realm. The Agreste family. His mother was lost in the war of the realms. That was before the realms were separated, hundreds of years ago. Adrien is 231 years old. Every vampire has their dark form of which they are not in control of. Gabriel Agreste's dark form goes by the name of Hawkmoth. Adrien's goes by the name of Chat Noir. However, all he wants to do is live his life and escape. He hates being a vampire. He wishes to be free and be good.

There isn't much to know about the mermaids except that sirens and mermaids despise each other. Mermaids use their voice to make people happy, sirens use their voice to manipulate people and trick people. Sirens are mermaids but who chose to be evil. Once they have done so, a mark is formed on their skin which allows you to tell the difference between a siren and a mermaid. Marinette Dupain-Cheng is a mermaid. She is the daughter of the King of The Vampiric Sea. The mermaids have kings of the 6 realms' sea but the mermaids can travel freely anywhere. All Marinette wants to do is walk on land.

Now to the story...

"Marinette! Marinette, where are you?!"

The sound of her father's harsh words echoed around the palace as he swam around the corridors looking for her. Marinette quickly swam out of her room and crashed into her father by accident.
"Oops.. sorry father," she said with a nervous chuckle as she helped her father.
"I have been looking for you for 10 minutes," he said sternly.
"I know and I'm sorry. I overslept again," she said, scratching her head nervously.
"Don't let it happen again. Your mother wanted you," he said with a sigh.
"Yes father. Is there anything else you need?" she asked, fiddling with her thumbs.
"Not at this moment," he said with a huff before swimming away.

"That was a close one," she said, letting a sigh of relief escape her lips.
"I'll say." Marinette turned around and saw a small jellyfish swim beside her.
"Sorry for hiding you, Tikki. You know my father's strict rules," said Marinette with a nervous smile.
"Of course I do. No jellyfish, turtles or crabs in the palace. No using your voice for selfish reasons unless you choose to be a siren where there is no going back from. No walking on land. No magic from evil people. And finally, no interacting with the two legged," explained Tikki, remembering the rules from the top of her head.
"Exactly. So let's get you out of here before he spots you," Marinette whispered.
"Thank you," said Tikki, nodding in approval.

The mermaid and jellyfish began to swim out of the palace and found themselves in a coral field.
"You should head back. Your mother needs you," said Tikki with a friendly smile.
"Alright. See you here tomorrow?" asked Marinette with a nod.
"Okay," said Tikki.

Marinette watched Tikki disappear into the distance before she swam up to the surface. She looked around and saw the coast. She didn't go near but she saw a house nearby.
"That must belong to the oldest vampire," she whispered with a smile of awe. "I wish I could see it for myself." Marinette quickly dived under the sea and swam back to the palace.

Meanwhile, a vampire of 231 years is waking up. He let out a yawn before stretching.
"Good morning Adrien," said a voice. He looked next to him and saw a bat sitting at his window.
"How long have you been here, Plagg?" asked Adrien with a frown before walking over to his friend.
"Not long. Just since sunset," replied Plagg, flying upwards to see his friend.
"Is my father home?" he asked.
"Yes. He wants to take you hunting," answered Plagg.
"But..."
"Sorry kid. Here he comes," whispered Plagg as he quickly hid in the wardrobe. The door opened and Gabriel Agreste came walking in.

"Father," mumbled Adrien sadly.
"Time to go hunting," he said, licking his lips.
"But we ate two weeks ago!" protested Adrien.
"Why are you fighting your nature, Adrien?" snapped Gabriel. "Now let's go. There's a new stock of humans in the butchers shop."
"Yes father," said Adrien with a sigh.

His father was the oldest vampire in the realm. The most evil of heart and the most deadly. There was no way that Adrien could tell him that he wanted to be good. That he wanted to rid himself of this burden. That he wanted to stop using Chat Noir. But he couldn't.

The two transformed themselves into their dark forms.
"I smell the blood from here," said Chat Noir with a grin, licking his lips.
"I know son. What do you say we get extra today?" said Hawkmoth with a smirk.
"I like that idea already," chuckled Chat Noir.

A purple mist surrounded them and transported them into the town square. They looked at the butchers shop and began to laugh maniacally.

After a few hours, the blood from the two humans had been drained.
"That should last us another two weeks," growled Adrien, glaring daggers at Hawkmoth.
"Why did you change back? The night is still young," asked Hawkmoth.
"I don't want to be evil. I want to be good!" cried Adrien.
"And I said no. For that comment, stay in your room for the rest of the two weeks," snapped Hawkmoth as the purple mist surrounded him and he was gone.

Adrien looked out the window and stared at the sea and it's waves.
"Mermaids... they can free me of this burden," he muttered.
"You can't be serious," scoffed a voice.
"Plagg!" cried Adrien. He opened up the wardrobe and shook his head playfully. "Why are you still here?"
"I got trapped," mumbled Plagg. "But you can't seriously expect good mermaids to help an evil vampire," added Plagg.
"Then I'll have to show them that I want to be good," said Adrien determinedly.
"Good luck," scoffed Plagg as he flew out of Adrien's window. He stopped and turned around.

"Good luck with your grounding," he said with a chuckle before flying away.
"Yep," said Adrien sadly.