This one is embarrassingly underdeveloped, but this is the basic idea: It's a forest located in Germany that, if an extended amount of time is spent in it, you slowly become more primitive and animalistic until you actually become an animal. What you turn into is based on your overall personality, for example a sneaky or furtive person (D-Class that was a thief?) would become a raccoon, while a hyperactive, twitchy person would become a squirrel.
Was your idea inspired by myths related to the Black Forest?
How long does it take for this transformation to occur on average? Does it stop if you leave? Does it start again if you come back? Is it the location itself that is the source of the anomaly? It could be something else, like microbes in the water or the plants.
What do you mean with "primitive and animalistic", exactly? More aggressive, for example? You should be careful as to not make it sound tacked on to grab the reader's attention.
How much of the victims' personalities, memories and intelligence remains after the transformation is complete?
I think it would be interesting if people remained intelligent and capable of speech and formed small, hidden communities of talking animals that may have inspired some fairy tales.