Inspired by "The Plagues" from The Prince of Egypt, and by the thought, "What if Chat Blanc, but Ladybug?"
When Chat Noir was stopped by Bunnyx, saying she Ladybug needed his help in the future, he didn't question the time-traveler. He'd seen enough Dr. Who to know that he had to keep his curiosity to himself, even though he really wanted a good look at what seemed to be a cyber-future Paris. But in this future-Paris he now found himself in, he was wishing he asked more questions.
The first thing that he really noticed was that it was quiet. Not just "nighttime city" quiet, or even "outdoor nature" quiet, but something more along the lines of "just before the big scare and you die in a horror game" quiet. The second thing he noticed was how cold and dark it was. The dark wasn't so much of a problem thanks to his night vision, but the cold began to chill him down to the bone. Those chills then began to run up his spine as he realized he wasn't stepping through snow, but volcanic ash.
Noticing the ash then brought his attention to something he had missed initially thanks to his night vision: in spite of how dark it was, the sky still seemed to glow. It took him a while to figure it out since he couldn't look directly, but eventually he did. Something was eclipsing the sun, and it wasn't the moon. He couldn't tell what it was, other than that it was, for lack of a better term, "fuzzy". Between the brief gaps in whatever mass was blocking the sun and the solar corona, it was almost as though the city was illuminated by a combination of the aurora and liquid refraction.
That was when he started to take stock of the city properly, and his hart simultaneously sank in dismay while also trying to leap out of his throat in terror. It was as if he had stepped into a post-apocalyptic world takin out of Disney's film, Sleeping Beauty. Black, thorn covered vines dominated the broken, abandoned structures all around him. Occasionally, one of the vines would be topped with a black rose. Small black/red motes also floated gently through the air, prompting him to quickly find a shelter and scavenge some cloth for a quick mask to avoid breathing in anything dangerous. He then began moving from structure to structure, trying to find some way to find out what was going on, and what had happened to his beloved city.
His baton failed him for the first time, as attempts to access the internet or any kind of news resulted in a fuzzy screen. Time travel and temporal displacement; a quantum disconnect, so the magic option was out. Most buildings didn't even have power. Occasionally, he would find a newspaper or magazine, but none of it held any hints of some kind of massive, dangerous Akuma, and were likely older than whatever Akuma had caused all this. Dread continued to pool in his gut as he began to question how it had gotten so bad. Had there been a big fight? Had they failed? What happened to the other Miraculous? What happened to him that required Bunnyx to reach so far into the past to get help from... well... HIM?
He eventually found an internet/LAN café that still had power, and thankfully still had a working computer as well. A quick check of records and key codes behind the desk, and he was logged in and testing the digital waters. Success! Wifi may have been down, but apparently an optical fiber cable was still undamaged. After some more scrounging for a toolbox and pliers, he was spliced in and finally accessing the World Wide Web. He quickly pulled up a search of the most recent news articles to see what he could find and figure out. The first articles that showed up were in English rather than French, but it quickly became apparent why when the first article's title and accompanying image of most of Europe covered in red appeared.
"Ladybug Plague" Advances in Egypt! Hawkmoth Still At Large!
The next articles followed much in the same line, emphasizing just how alone he was in not just Paris, but in all of France. Even in all of Europe! He began looking at more articles. Older ones too. Skimming them as quickly as possible to try and find where it all began. The articles' titles burned into his mind as the minutes went by.
Russia Prepares "Scorched Earth" Contingency as Plague Spreads!
Plague Refugee Crisis Grows in India, China, and Australia!
Contact Lost! UN's "Counterplague" Coalition Fails!
Black Roses Discovered on the Shores of New York!
Another Hero Falls! Camilla Hombee's Body Recovered!
Then he finally discovered what was probably the only French article, and with it, a name.
Lady Plague Demands Hawkmoth's Surrender!
At that point, Chat found that he could no longer continue his search, because he could no longer move, and was instead being dragged out backwards into the street, and then into the air. Claustrophobia began to set in as he realized he could only move anything above his neck, but that was enough to let him see what was happening. What he had originally thought were "motes" in the air were actually ladybugs. Thousands of small, red and black, glowing insects that had slowly collected on him like a miasma until they were strong enough to restrain him and fly him to the Eiffel Tower.
That was when he saw her.
She was older (time travel, again), and she was different, but he knew it was her. Her red and black spotted suit was replaced by a pure black one with red-stencil rose motifs of roses, and instead of ribbons, her hair sported two roses: one red, and the other black. She turned to look at him, her acidic green gaze carrying none of the warmth her natural blue should.
But the most terrible thing he saw was the familiar, inactive ring on her left hand. It suddenly made a lot more sense why Bunnyx needed him.
"My identity has been compromised, M'Lady. My father knows."
"He's Gone, Marinette!"
"Today, all of Paris mourns as Adrien Agreste, also known as Chat Noir was laid to rest. The funeral..."
"It's your fault! I hate you, Ladybug!"
"Lady Blight, I grant you the Power of Unlimited Creation. Together, we can bring them back! But there is something I require something from you..."
"You will never find rest, Hawkmoth. I will find you."
Bunnyx flinched when she saw Chat Noir get manhandled by a swarm of "Blightbugs", but when she looked away, she saw another window get blocked by a tangle of black thorns. She shuddered and turned her attention back to the immediate window. So far, the vines had yet to breach her Burrow, but she didn't know how long that status quo would last.
AN: For those who are wondering, yes, just like how Chat Black had unlimited destruction that cracked the moon in half, Lady Blight has unlimited creation, allowing her to create a swarm of "blightbugs" so large that it's eclipsing the sun.