The Fallen 2: Aerie and Reckoning by Thomas E. Sniegoski
Aaron and Camael finally stumble onto Aerie, a peaceful town where the fallen angels and nephilim are hiding away from the powers. While in Aerie, Aaron stumbles upon information about his own father, while trying to battle Verchiel and the Powers. Vilma also joins him in her nephilim form, which comes as a surprise to him. In the second part (Reckoning), Aaron finally meets his father and helps to save the world. Verchiel wants to release hell onto the earth but is finally dealt with and sent back to heaven to seek absolution.
Reckoning was a little more violent and gross for me (too much blood and gore) but I enjoyed the storyline of Lucifer being Aaron's father and not being what the human world has depicted him to be. The idea of hell residing inside Lucifer was an interesting idea.