The book I am looking for is a fantasy book which was, I believe, published in the 2000s. It was supposed to be part of a trilogy. It took place in a alternate/different world other then Earth and focused on a protagonist that was meshed/combined with a demon. Something akin to Dr. Banner/Hulk. In other words, he was either one or the other. The story was based around the boy turning into the demon and going on a journey to get rid of it. One of the details I remember most was the cover art which was a bigger then average man but in demon form and very black.
This book was loaned to a friend and simply can not remember title or author's name and would really like to find out if there were any others published after this.
Thanks for any help.