I usually think that Plagg was hiding the truth by saying he ate too much cheese to cover it up. Maybe it could be that it was too chaotic to tell the future generations. So I made up how it really happened. 

Characters belong to (c) Zag.

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It was 480 B.C. when a Greek warrior with the black cat miraculous from Athens came to the city of Atlantis for an exchange with Pandora's box.

On his shield was the paw logo and his helmet had cat ears. Once he approached the King of Atlantis, he took his helmet off and bowed.

"Your majesty. I came to you with a gift for the exchange of the trident" The Cat Greek warrior said.

"And what might the gift be, young warrior?" The King asked.

"I give you the Pandora's box. But there's one catch about it" The Cat Greek warrior shows him Pandora's box.

"What's the catch?" The King asked.

"Never open it" The warrior warned.

The queen did not trust this feline warrior like it was some kind of a trap. Thankfully she has the miraculous of the Dolphin with the power of foresight. Her king holds the miraculous of the shark.

"Wait a minute" She stood up.

"Sonar, jump up!" The Queen transformed into her dolphin-themed heroine, "Forsee" She said the power word, and her eyes glowed blue to see the future. She saw the King opening the box despite the warrior's warning, influencing the King with madness.

"What's going on?" The Cat Greek warrior asked.

"You brought great danger to Atlantis!" The Queen roared.

"That's nonsense! It's merely a trade to ensure it won't cause chaos in the world!" The warrior protested.

"My foresight never lies!" The Queen yelled.

The king thought his queen was being cranky like always and when he gullibly opened Pandora's box, it unleashed the fury creatures out to cause havoc. All had sorrow, disease, vice, violence, greed, madness, old age, death.

"No! What have you done?!" The warrior exclaims.

Fury creatures were causing chaos everywhere, feeding on every negative emotion from the Atlanteans. The king should've listened to his queen and now he wants revenge on the Greek warrior. He whacked him with his trident, causing the cat warrior's ring to fall off, making him detransform.

"You shouldn't have brought that box here! Now you will perish for this chaos! Chompp! LET'S FEED!!!" He transformed into a shark-themed warrior.

"Oh, boy. I don't want to do this but.....CATACLYSM!!!" Plagg used his destruction and it caused a mighty quake across the ocean to trigger a tsunami miles away from here.

"NO! WHAT HAVE YOU DONE?! Chompp, finish eating!" The King detransformed, handing his necklace to his kwami, "Get it to the shelters! Avenge us!" He told him.

Chompp nods and as he glares at Plagg, his scleras turn red, "We will meet again, Plagg! In another time!" He floats away into the chambers below Atlantis.

People screamed at the huge total wave coming and Plagg saw his holder that could not outrun it. There was no escape. Temples were collapsing one by one, nearly killing Atlanteans who were getting crushed by debris.

"Plagg, you must go! Now!" The Greek warrior said.

"But you'll drown!" Plagg said and the jets of water sprout out from the ground, causing cracks everywhere.

"Now! You can make it but we all can't!" The Greek warrior said, accepting his fate of death, handing the ring to him, "You must find a new holder. A worthy one! Now go!"

Plagg flew up as the whole place sank into the sea. He looks down, knowing he'll live with the guilt for 2500 years.

(Present day: Paris, France)

With a new holder of the butterfly miraculous is now in the wrong hands, Team Miraculous has once again defeated an akuma. It's been 6 months since they still cannot find her ever since Ladybug and Cat Noir discovered the world underground with Blu Tembo and Panthera.

"Good job, guys. We'll find the monster and put an end to this Akuma plague. Right now it's time to get back to our lives. Bug out" Ladybug swings by.

Cat Noir had been having these weird dreams for a week like it was some kind of message or warning whenever he is transformed for a cat nap.

"Dude, you okay? Skipped a cat nap again" Carapace asked.

"Huh? Yeah, I missed them. I might need a glass of milk" Cat Noir said.

"Same dream again?" Carapace asked before going into the alley to detransform together.

"Of course. Plagg claws in!" Cat Noir became Adrien again, "It's like both a nightmare and dream at the same time"

"Like what? Like when Nightmentor gave you a nightmare headache of whatever it was you were afraid of?" Carapace guessed before detransformed and fed Wayzz a macaroon.

"Yeah, that I might get the power to destroy the world as Anti-Cat. But this feels like it happened like thousands of years ago or something" Adrien said.

"Maybe from a previous cat holder before you. Dude, you need to calm down. You've been stressed out for a week and you also have your number one fan posting all of your stories of fighting previous akumas with your lady. You need a break now" Nino said.

"I know I know. I just sent my video to her now after we defeated an Akuma 30 minutes ago. I'm just getting a feeling it's more than just a dream" Adrien said.

"And I am more than hungry for cheese, please. All this teenage talk makes me barf" Plagg said, getting impatient with cheese.

"Here you go" Adrien hands him some camembert, "I must find out more about what's going on," Adrien said.

"You know. Some dreams could be prophetic or some kind of message. Are you sure you haven't had a lovely happily ever after with your dudette in your dream?" Nino asked.

"I wish I dreamed about our future," Adrien said, looking at the sky.

"Look at my man growing up so fast to become a knight for your princess" Nino teased.

"Heh, thanks" Adrien chuckled.

"And that dinner date with her last night really made you something. And yet you still haven't thought of what you want to do in life" Nino said, walking with him in the streets.

"Maybe propose to her once I've graduated?" Adrien wondered.

"Sounds like something," Nino said.

-

With Pelle, he was sitting at the Café, reading some articles on his iPad. He got a call from Luna through FaceTime as he answered, "Hola, amiga. How are things in Costa Rica? Jaguars behaving well?"

"Indeed, Chico. What about your elephant children?" Luna asked.

"Same as always. By the way, I saw your new cat blog again. I'm loving it" Pelle said.

"Thanks. It's hard to keep track while protecting Costa Rica and stuff. What about you? Stopping poachers and mercenaries in Africa again?" Luna asked.

"Yep, even helping out orphaned calves," Pelle said.

"Aww.....that's so cute. You should have your very own sanctuary. Blu Tembo's Elephant Sanctuary" Luna imagining a sign title.

"Maybe one day," Pelle said.

"That would be something," Luna said.

"Oh, love to chat but Sweden needs Blu Tembo again. Talk to you later, love. I mean amiga" Pelle quickly corrects himself.

Luna just laughed a little, "Go kick some ass, Chico. I also got another duty again. Adios" Luna hangs up.

Pelle shook his head and then drank his latte.

-

Cat Noir was still taking a cat nap again on the rooftop like a cute little kitten. While he was sleeping, he could hear the screams of people and a tidal wave approaching. Cringing at the thought of it as he squirms in his sleep. Hearing the swords clashing everywhere like a war and the people screaming for their lives. Then there were crashing sounds and more screams, making Cat Noir jerk up awake.

"Shit!" He nearly spooked himself, "I need to know what's going on with me," Cat Noir said.

"There you are, kitty. Cat nap again on the roof. Are you still leaning into your cat tendencies?" Ladybug came with a milk carton for him.

"Hey there Bugaboo. Thanks" He accepted the milk carton.

"I'm worried about you, kitty. It's like you haven't gotten any sleep" Ladybug said before having her own milkshake.

"I'm fine, milady. We cats are just having a bit of hairballs coughing out" Cat Noir lightens up the mood.

"If you say so. It seems to happen frequently these days" Ladybug said.

"Come on, I'm just adorable like a sweet kitten" Cat Noir trying to look adorable.

"Awww.... so adorable" Ladybug scratched his chin.

"Puuuuuurrrrrrrr" Cat Noir purred and nuzzled her hand.

Ladybug couldn't help but chuckle at how cute he sounded, "When was the last time you were petted?"

"By Marinette" Cat Noir said.

"Your favorite princess you always swing by?" Ladybug asked.

"Yeah, she seems so happy with her prince but loves to keep me company too," Cat Noir said.

"My poor kitty," Ladybug said.

"I'm doing good, Bugaboo" Cat Noir said.

"And your kwami behaving as usual? I can still smell Camembert cheese on you" Ladybug said.

"Yeah," Cat Noir nodded.

"Oh, kitty. What am I gonna do with you?" Ladybug teased.

"You said I'm your favorite, both your partner and kitty" Cat Noir nuzzled on her.

He could smell that she had been at the bakery, "Mmmmm.... You've been at the bakery?" He guessed.

"Uhhh....yeah! Exactly!" Ladybug quickly said, nervously.

"Thought so, their macaroons are so delicious there," Cat Noir said.

"As much as you like milk too" Ladybug made him drink milk from a carton.

"Mmhmmm" Cat Noir said while drinking.

"Have you seen the new post on your cat blog?" Ladybug knows that he enjoys Luna's blog too much.

"I have. Carapace, you, Rena Rouge and I kicking ass together" Cat Noir said.

"And you're the only one with nine lives, kitty. I bet you just waited a long time for a big fan to admire" Ladybug scratched his chin.

"Wild cats have emotional needs" Cat Noir moved her hand away.

"I know," Ladybug said.

"Now I must go. A kitten has to get back to his mother" Cat Noir meowed.

"See you later, kitty cat" Ladybug pecks his cheek before swinging away.

-

As he came through his open bedroom window, he detransformed and slumped on his bed, exhausted. These dreams he was having were driving him crazy. Could kwamis carry memories of the previous holders to pass it down to the next? He held out his kwagatama to find answers from the previous Cat Noir holders before him.

He took a deep breath before asking, "Show me the history of Cat holders"

Nothing happened since it only works on being touched to the symbol of the Order of the Guardians. Like on portraits or anything with a symbol.

"Kid, remember that I told you to take a picture of that portrait at the museum?" Plagg held the printed photo of the previous cat holder warrior with the symbol.

"Oh, is that why you nagged me to do that yesterday?" Adrien pouted.

"Yep, Joan Arc's lover," Plagg said.

Adrien grabbed the printed photo and used his kwagmata to touch the symbol, releasing all the previous black holder icons for the holder to choose from. Adrien reached and touched Dark Grimalkin, summoning his ghost image of him.

"Good evening. What is your name?" The knight asked.

"Whoa, I'm talking to a ghost?" Adrien said.

"You're talking to a memory of Dark Grimalkin," Plagg said.

"Plagg, so good to see. Ah, thou hast a new holder. Most young as a kitten and yet most charming" Dark Grimalkin said with old English.

"Yes, that's right. I'm looking for answers" Adrien said.

"It is I, Dark Grimalkin, young lad" Dark Grimalkin introduced.

"Okay, I just need to know something, I've had these strange dreams of hearing screams," Adrien said.

"Has a sorcerer put an evil spell? A warlock? A prophecy?" Dark Grimalkin held his ax.

"I'm not sure, but it feels like a cataclysmic event," Adrien said.

"Pray tell, kitten," Dark Grimalkin said.