Well, first try pitch shift. I've done voices like that in the past and the best effect you can do is drop the pitch of the vocal low enough so that the tone is almost demonic but the words are still understandable. On top of that it's just a matter of coloring, all experimentation. Reverb will give it that "big" sound, and it sounds like there is some flanger on that voice from fearfair.com. The main thing though that makes the entire vocal is the dropping of the pitch, that and some good voice acting.