Even Miraculous users aren't immune to becoming akumatized in Miraculous Ladybug. Season 3 introduced viewers to Cat Blanc, the akuma version of Cat Noir, a transformation that comes from heartbreak caused by Marinette and pressure from his father -- and it all started with a gift.
How Cat Noir Is Created - and Defeated
In the Season 3 episode of the same name, Cat Blanc is revealed to be the future, akumatized version of Adrien's superhero persona. In the present day, Marinette had left a gift for Adrien in his room as Ladybug but signed the card from Marinette. As she was leaving, Adrien caught her, then saw the signed card, which led him to figuring out that Ladybug and Marinette were the same person and realizing that she loved him. After telling her that he loved her as well, they began dating, which was so big that it even became a news story.
Unfortunately, Adrien's father Gabriel (the villain Hawk Moth) disapproved of the relationship, saying that Marinette was giving his family a bad name. He threatened that he would pull Adrien from school unless she broke off the relationship, and not wanting to have Adrien cut off from his friends, she obeyed. Gabriel, as Hawk Moth, then sent an akuma to try and change Marinette in her heartbroken state, but Adrien blocked it, revealing that he is Cat Noir not only to Marinette but to Nathalie (Gabriel's assistant and holder of the Peacock Miraculous) and his father as well.
While fighting Hawk Moth, he calls Cat Noir by his real name and even reveals his motives for going after the Miraculous and his identity as Adrien's father. Taking advantage of his son's confusion as to what to do, Hawk Moth akumatizes him, resulting in Cat Blanc's creation. His conflicting emotions lead to his Cataclysm overloading, causing an explosion that destroys Paris, effectively killing both Ladybug and Hawk Moth.
Cat Noir's transformation into Cat Blanc in the present is prevented though, thanks to Bunnyx (the holder of the Rabbit Miraculous) who traveled into the past to prevent the destruction of Paris. She found Ladybug and took her into the future to defeat the akuma possessing Cat Noir. He begs her to give him her Miraculous, and the two fight, with Ladybug breaking the bell around his neck where the akuma was hiding. This transforms him back into Cat Noir, and Bunnyx takes her back to the point where she signed the card on Adrien's gift. Ladybug erases her name and tells Adrien that she was delivering it for some Brazilian fans because there was a postal strike taking place. This causes her to forget about Cat Noir's true identity thanks to the powers of the Miraculous.
How Powerful Is Cat Blanc?
As Cat Blanc, Adrien can cause massive explosions using his Mega Cataclysm ability. These explosions are what caused the destruction of future Paris, and could even destroy the fabric of the universe. He also can launch long-range attacks as either a beam or a blast. Not only this but he isn't limited to attacking one thing at a time like Cat Noir. Instead, his destructive abilities can be used on multiple targets without his power source draining and his transformation reversing. It's clear, knowing this, why Bunnyx was so keen to stop something as seemingly innocent as a letter from coming between Paris' beloved superhero team.