Here's the story you are looking for, The Darkness Within by Kurinoone.
http://www.hpfandom.net/eff/viewstory.php?sid=24500
The sequel called A Part of Me
http://www.hpfandom.net/eff/viewstory.php?sid=25693
and the final part called Deepest Reflections
http://www.hpfandom.net/eff/viewstory.php?sid=27508