For my fic, I want to involve some permanent transfiguration. I.e. I have bad guys who will non dark magic to make a kind of precise chimeras of humans. They would have tails, ears and some other carnivora traits semi-randomized.
It must be fairly chimera like - cat ears, a binturong tail, bear fur.
Now, I was thinking it mostly comes from failed polyjuice and animagus transformations. And because they're all mixed in together, it's difficult to remove traits one by one and you must know how they all went in together. -> it must be clear that you cannot do what Pomfrey did to Hermione in book 2.
There must be an eventual way to control it, but it should last longer than 10 years even with Dumbledore looking into it. I have a whole transfiguration layer theory that includes things that look like cryptography, But that would be quite outside the scope of my fic. So - what could be a reason that 10 years still is not enough for this magic to be reversed?
Why not dark magic? I mean, if I have to, but the villains of this story aren't big fans of dark magic.