These transformation stories are one of my favorite types, where the main character consensually with a little mental prodding (or none at all) decides to do something they should not and that leads to the transformation of their body and sometimes their mind.
Here’s two that I have returned to several times when I’m in the mood:
http://agrowingconcern.com/personaldemon.html
And two from the site
Do you guys have any similar stories that get you keep coming back?