Perseus Jackson Monster Shifter
I found this fanfic on fanfiction.net and decided to show it to the Wattpad community. All rights to the autor of this fanfiction: Hans50; from whom I got permission to post this story here. He's an awesome writer and you should go check his account and his other story: Betrayed Son of the Sea.
Perseus Jackson is trapped in Tartarus. After the council of the gods, save a few, betray and sentence him to eternal suffering, something happens to our hero that will change him forever. He is tortured and cursed. With a horrible curse placed upon him that turns him into the things that he hunts and despise, will he suffer in the pit or will he thrive? He is the Monster Shifter.
(By the way, this is not a Percabeth story. Sorry, Percabeth fans...)
DISCLAIMER: I don't own this story, Hans50 and Rick Riordan do.