Miraculer left me hungry for world building so here, have at it. How I reason out cataclysm's effects.
I posted this on my Tumblr a while ago and just sort of forgot about...haha

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"So," Adrien says, flopping onto his bed and burying his face into the pillows. "That's what a cataclysm feels like."

It's been a few hours since they had defeated the akuma. Since Ladybug had cast her magical cure and reversed all the damage. He's been healed for a while now and can say with confidence that the mark no longer hurts. No, he doesn't hurt anymore. He aches.

An ache like nothing he's ever felt in his life, different from fatigue or strain. It cuts down to his very soul. An ache he feels so deeply he isn't sure just where it starts and where it ends.

"Yeah no. Not so much," Plagg says, he's followed Adrien's example and thrown himself down onto a pillow. Close but far enough not to touch.

"What." Adrien's voice comes out muffled.

"I wouldn't really know, Tikki would," Plagg sighs, he never likes to have this talk with his kittens. "But kid, Cataclysm has the power to destroy a Miraculous. When used on a person, seriously used on a person, not just the watered-down version that Hawkmoth's akumas can produce things go much much differently."

"You mean it's worse." Adrien can't really understand how it could be. Even with the power of Ladybug's miraculous cure he still feels the phantom sensations of the magic snaking its way through his body. He still has the black tendrils localized at the point of contact and branching out, reaching and reaching toward his heart.

"Despite how it may seem the Butterfly does have limits." And God, if Plagg isn't thankful for that. "One of those limits just so happens to be the fact that it can't truly duplicate the power of another Miraculous. The magic just isn't there."

Adrien lifts his head and looks up, meeting Plagg's eye. "What would happen than if I used cataclysm on a person."

"It depends kid." It did, Plagg's seen it used so many different ways on the body. Every time he thinks he's found the limit, every time he thinks that this has to be it, he'll get a kitten that surprises him.

It's never a nice surprise.

"On what?"

"A lot. On intent. On target. On focus. There's any number of things that affect how cataclysm reacts to a human body. The corruption, that's not it. Trust me."

Adrien drags himself up into a sitting position. This is serious, the sort of conversation that requires every ounce of your attention. "So you have to really mean it then, for Cataclysm to kill someone. You have to really want that person dead."

"You have to mean it. I just have to be careless. The dinosaurs really were an accident, you know." That's the greatest difference between the two of them. Chat Noir is destruction concentrated and focused on a single thing. Plagg, he's destruction in its purest form given shape.

"Why would Ladybug's kwami know what your power feels like?" Adrien asks.

"Look, kid," Plagg sighs, trying to think of how to phrase this delicately. He's not a teacher or at least not a good one. Tikki has firmly held that role for all the millennia he's known her.

"Plagg why would Tikki know what it feels like for her holder to be cataclysmed if it hasn't happened before." It's the logical conclusion to come to, the only one that makes sense.

That doesn't mean Plagg is particularly thrilled to explore it.

He will. Because he opened this can of worms. He just won't be happy about it.

"Do you remember what I told you when you first opened the box." Plagg has been giving the same speech for the last four centuries. Particularly once he realized just how clueless those who opened his box were.

Would it be too much to ask the guardians to…I don't know attach a note.

The answer was apparently yes. It was too much to expect them to inform the chosen exactly what they were chosen for.

"You said that my job was to protect the Ladybug," Adrien says, that's the thing Plagg has always stressed. No matter how lax the kwami behaves he has always stressed that Chat Noir's job above all else was to protect the Ladybug.

"Yes well, not all my kittens have taken that to heart."

Plagg thinks he's gotten better over the centuries at pushing this. The look of genuine confusion that crosses Adrien's face makes him wonder if he's gotten a bit too good at it.

It's certainly bred some...less than ideal habits into his kitten.

"What do you mean?"

"I mean that my cats have killed just as many Ladybugs as they've protected."

"The Ladybug and Black Cat are two halves of a whole, they can't be awakened without each other." Plagg shakes his head and takes a deep breath, "Out of all of us, my Miraculous has been misused the most and because of that Ladybugs have found themselves facing Black Cats across the battlefield more times than I would like."

The color drains from Adrien's face, he feels sick. "You mean Cataclysm has killed a Ladybug before."

"Cataclysm has been used to do much more than to just kill a Ladybug." Plagg crosses his fingers and hopes that this won't upset his kitten too much.

There is a long bloody history between Miraculous.

The Ladybug and Black Cats especially.

"Does Ladybug know about this? Is this why she's so against revealing ourselves, she's scared of me," Adrien says, he hopes not.

He's not entirely sure how he can bring himself to face her come the next battle.

"You have to understand kid…what some of my past holders have done…I'm destruction in its purest form and destruction is a temptation that's hard to ignore." Plagg wishes he could lie.

He can't though, not about this.

"So she is then, she's scared of me." It would make sense. It would explain so much about the distance she keeps between them.

Understanding does little to ease the knots that form in Adrien's stomach.

"No no, she's not." Plagg's out of his depth. He's not a comforting presence, he knows. But his kitten looks so crushed…he can try. "She trusts you with her life, anyone with eyes can see that."

"Then why is she so against knowing who I am?" Adrien's voice has gotten thicker. He doesn't want to cry over this. He doesn't…it just hurts.

It hurts to think that after everything they've been through she would fear him. That's the absolute last thing he would ever want from her.

"That's Tikki's influence. She's lost so many bugs in some of the most gruesome ways imaginable." Plagg shakes his head the memories of all the Ladybugs he's helped to kill are always just there, under the surface but there.

"I would never." Adrien's voice is quiet barely above a whisper.

"I know you wouldn't. Ladybug knows you wouldn't. And I'm sure even Tikki knows. But we've been around humanity for a long time and old habits die hard."

"Have things...have things really gotten that bad."

"You know cataclysm relies on intent right. Well, not all my cats have wanted to kill with it. Some..some just wanted to use it to hurt." Some of his holders had made death seem like a kindness.

"So Cataclysm can be used…." Adrien gulps and trails off reluctant to say the word.

"To torture, yes. It can and has been used for torture more than once."

"Oh God." Adrien hunches forward and buries his face into his knees. He couldn't imagine what it must be like to want to hurt someone that badly. To hate someone so strong that death would be a mercy.

He can't even muster those feelings against Hawkmoth.

"Kid! Kid listen to me," Plagg says, coming to rest on top of Adrien's knees. "It hasn't just been used against Ladybugs, it's been used against some of the worst people to ever come into existence."

"That doesn't make it better," Adrien chokes out.

"Maybe it does, maybe it doesn't but kid times have changed a lot over the last couple of centuries. Death on the battlefield, it wasn't nearly as big of an issue as it is now."

"But still how could…how could any Black Cat want to hurt their Ladybug." There's just the smallest bit of wonder in Adrien's voice. He can't help it. He just can't imagine ever turning against his partner.

"To be fair, while it hasn't happened nearly as often it's still happened. Not all Ladybugs have been saints. Tikki has been misused as well, arguably with even larger consequences than when I have," Plagg says.

"What's it do. When it's used on someone?"

Plagg doesn't want to tell Adrien, really he doesn't, but he's started this conversation and he has to see it through. "Like I said it depends. The more malice the stronger the cataclysm…it depends on what you want. Do you want to kill them? Do you want to make them suffer? Where do you want to effect? All that changes things."

"What does it do Plagg." Adrien's voice is sharp.

"It turns them to dust. Eats away at them till there's nothing left. Like I said it varies. When used to torture I've seen it rot down to the bone. I've seen it sever limbs and peel away skin. Once…once I even saw it erase someone out of existence." That had been a particularly memorable use.

A cataclysm so strong that it was as if they had never existed, the very memory destroyed alongside the body.

"I don't-I would never," Adrien says, even though deep down, he's terrified that he could.

"No, you wouldn't. Kitten, I know you wouldn't. You're a good Black cat. One of the best." Plagg comes to hover up to Adrien's face. "You have to really mean it to use cataclysm like this. It's not something you can do by accident, you have to want to hurt someone, want to make someone suffer down to the very core of your soul. That's actually one reason the guardians started choosing children younger and younger to wield the Miraculous. Kids just don't have the malice that adults grow into. You don't have that in you."

"You're sure?" Adrien wipes his eyes on the sleeve of his shirt.

"Positive." And really Plagg is.