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Riddle is Master Fu and guardian of the miraculous: rare magical artifacts passed down from the founders, protected by the last of their descendants. He tried to wield them to his own twisted means in his youth, when he accidentally created Voldemort and has been trying to deal with the mad construct ever since.
Magic still exists, as does pureblood politics, the SOW party, and therefore, the Ruse.
The most powerful kwamis, the ladybug of creation and the black cat of destruction, have been lost for centuries; Riddle desperately wants to track them down
The ladybug miraculous is dormant: it has been passed down from rogue to rogue of the Lower Alleys, and appears but an ordinary pair of ceremonial earrings.
The black cat miraculous, also known as the Jewel of Dominion, is unearthed by Pettigrew, who is unable to wield its destructive power - instead, it turns inwards, and eats him from within. Pettigrew kidnaps “Rigel” in order to harness his magical power and gain control over Dom, but that goes around as smoothly as it does in RBC canon.
Rigel’s suppressor lies shattered on the ground, her magic swirling around her. She stares at Pettigrew’s corpse, a shredded husk by his own doing, the resonance about to consume her alive, thinking that this miserable tomb might be the last she sees. Suddenly, the Dominion Jewel wraps around her finger to coalesce into a simple crimson band - not unlike her suppressor once did. It shields her core and blocks the resonance, but at a cost.
Rigel doesn't realize quite how steep that cost would be until a tiny black cat winds its way out of her ring the next morning, demanding tea and attention. Ordering her around. Announcing a new role, a new title, a new mask to put on: that of Chat Noir.
The last thing Harry wants is more power, but once Dominion has chosen, he is harder to remove than not. Dom proves to be her worst nightmare come true. Rather than suppressing, it amplifies her power, and it’s hard for her to make her magic do anything but destroy and overwhelm. Her worst fear is her magic, her ruse, the weight of all her lies hurting those she loves; now she fears that the black cat’s cataclysm might doom them all.
With the awakening of the black cat, the Ladybug Miraculous also awakens from her long slumber - right into the hands of one Lionel Hurst.
Leo, like in canon, is hopelessly in love with Harry Potter. But that Summer, as he works out the kinks of his new title and new role as the vigilante protector Ladybug (on top of the Rogue) of the alleys, he gains a new partner: the mysterious Chat Noir.
Harry, as Chat Noir, just wants to get this over with so she can control her magic enough again to get on with her potions research. Her partner, Ladybug, proves an immense help. He’s charming, kind, patient, and a beacon of miraculously good luck to her endless misfortune.
The akumatizations start slow, but ramp up quickly. Dom's awakening did more than awaken Zuriel (the kwami of creation), it awakened Voldemort and the Papillion as well. Voldemort strikes in the alleys first, wreaking havoc as Ladybug and Chat Noir desperately try and master their new powers before its too late.
Slowly, they realize that their powers work best in tandem with each other. Leo grounds himself in his vigilante partner while pining for Harry, who seems more distant and busier this summer than ever. Harry learns control over countless painstaking duels in the alleys with Leo, while gradually falling more and more in love with her gentle partner with every new akuma sent by Voldemort. Only around Ladybug does Harry feel fully in control, and only around Harry does Leo feel fully himself.
Tensions rise more and more, and Ladybug and Chat Noir are soon needed in more than just the lower alleys. Akumatizations start happening in all of Britain, in wizards and witches from all walks of life, blood purity, magical alignment, etc.
The Ruse grows more and more complicated, especially once Harry has to return to Hogwarts for school, but the akumatizations speed up rather than slow. (She has to learn to apparate rather quickly, due to that.)
Archie figures out Harry is Chat Noir first - he needs to know, for the sake of the Ruse. Harry starts getting worn ragged with the effort of keeping the layers of their ruse(s) alive. Riddle finds out that “Rigel Black” is Chat Noir somewhere along the line and attempts to force her under his mentorship as guardian of the miraculous. This time, she can't exactly refuse. He doesn’t find out that Rigel Black is Harry Potter until far, far too late.
(When Rigel Black vanishes, only Riddle knows he's stuck around as Chat Noir. Harry might have dropped Rigel’s ruse, but she can't drop the black cat's curse. Every akuma attack after that feels like an inch away from a lifetime in azkaban. Harry loses the comforts, the friends of Rigel Black, but she feels his dangers every day she slips on Dom's sleek mask.)
On Leo’s side, only Rispah realizes that he’s the Ladybug for the longest time - and she finds out from Ladybug’s obvious crush on Harry Potter, of all things. It’s cute...until she realizes just how much of a burden these akumatizations are on the already too-busy Rogue, and she helps in any way she can. The Alleys mobilize behind their heroes, their Ladybug and Chat Noir. They may now be the heroes of all of Britain, but in their hearts, they will always have been of the alleys.
When Archie gets akumatized, his healing becomes the deadliest weapon, his form a mirrored haze. Every word and blow strikes true - Archie knows Harry too well to hold back. Chat Noir can barely fight, and they only win when Archie manages to throw off the akumatization right as he's about to reveal the Ruse for good. For the first time, Ladybug and Chat Noir realize they might not be able to complete this alone.
Eventually, Rigel convinces Riddle to entrust more Miraculouses into the hands of worthy wielders - those that prove a certain level of mental fortitude to resist the Papillion's call.
Archie becomes the fox; Hermione the turtle. Draco is, predictably, the Dragon, but his powers prove destructive on far more of an emotional level than any physical flame. Pansy is entrusted with the rabbit. Leo gifts Rispah with the bee, Marek with the monkey, and after much deliberation, Harry presents Caelum with the snake (although there is one unfortunate and awkward akuma attack where Leo attempts to gift Harry with the snake, and another where Harry gifts Leo with the lion, and no fight really is the same without the Black Cat and the Ladybug both at the forefront, is it?)
I don't have a super clear idea of where this goes, whether they all band together for a final dramatic showdown with Voldemort or if he is taken down bit by bit - whether the ruse, their identities, or both fall before or after or all at once; only that it is equal parts devastating and satisfying to see them all come together in the end, to see Leo and Harry find themselves in each other, no matter what form they each take on.