it's starting to feel like he's becoming a beast of Gaia or a beast candidate.

This looks like a word for word copy of A Demon Lord's Hero by Fahad09 on fanfiction dot net including the summary that has since stopped being updated as of 4 years ago.

I don't see anything acknowledging it but this fanfic was written by Adrian King1 on fanfiction.net called Game of Shadows that's already completed.