He doesn't know what happens. One minute he's struggling against the leash spell, the next he's tripping on his own momentum as the magic suddenly snaps, as he tastes freedom for the first time in- in ever, probably.
Adrien doesn't stop long enough to think about where he's going or why. He just bolts.
(Marinette's spell goes wildly off-target, but it's a stroke of luck for everyone involved that it does).
What Worse Luck is a Miraculous Ladybug Alternate Universe fanfic by GuardianKarenTerrier. In this fic, Adrien Agreste has lived for as long as he can remember under a curse locking him in the form of a black cat. While his father can cancel the curse and return him to human form, the curse reasserts itself as soon as he leaves the property. Despite that, Adrien still runs away every chance he gets... and as his treatment in his father's house is gradually exposed, it becomes very clear why.
- Adaptational Villainy: Gabriel Agreste, even compared to his portrayal in canon Miraculous Ladybug. While his canon portrayal is a supervillain who thinks nothing of unleashing apocalypse-level attacks against Paris on a daily basis, he still shows glimmers of humanity in his desire to bring back his wife and in his occasional interactions with Adrien. This version of Gabriel, on the other hand, confines his evil to his treatment of Adrien, and yet the sheer depth, breadth, and thoroughness of his abuse of his son far outweighs anything canon Gabriel did.
- Audience Surrogate: Part of Gabriel's mistreatment of Adrien was a complete lockdown on any except very carefully regulated sources of information. In particular, he knows absolutely nothing about magical law or theory, and so needs to have even the most basic things about magic explained to him.
- Central Theme: Consent is a huge theme in this fic, on many levels.
- Dark Is Not Evil: Plagg and Adrien both have the magical domains of destruction and misfortune. Plagg is quick to insist that these powers aren't automatically evil, but even he has to admit that they've got a bad reputation.
- Familiar: Varies a little from the standard trope.
- Most familiars are spellcasters in their own right. The bond boosts both parties' spellcasting, and if the familiar has a wild talent the mage gets access to it as well. In addition, a familiar and their bonded mage can always understand each other, regardless of species or language.
- Familiars do not have to be non-human. In fact, it's apparently possible to have a non-human mage bond a human familiar.
- The bond between mage and familiar is always supposed to require mutual agreement. Some creatures (such as cats) are impossible to bind without explicit consent by both parties, while for others it's possible but highly unethical.
- Familiar contracts are usually written down in the setting's present day. Plagg and Tikki's contract was made verbally, but that was both because it was a long time ago when the traditions were different, and because they had to make their pact in a hurry for some reason.
- Hidden Agenda Villain: It's all but certain that Gabriel's abuse of Adrien must have some purpose beyond sheer sadism, given the amount of effort and resources he has put into it and the risk he runs if even a fraction of what he's done comes to light. What that purpose is, though, has yet to be revealed.
- High on Catnip: Played for Drama. Apparently, Emilie once give Adrien catnip to pass the time while his father was getting ready to cast something on him. Adrien only has very vague and disjointed memories of what happened next, but neither he nor, as far as he knows, anyone else ever saw his mother again.
- I Choose to Stay: Plagg apparently decided at some point that he prefers life as a cat to life as a human. His contract with Tikki includes a clause to prevent her from breaking his curse (even by accident), and he makes himself scarce when Marinette is casting her Miracle because she's strong enough that she might be able to break the curse without realizing it.