I believe this is played on a 6 string tuned down seven half steps to ADGCEA, though it may be on a 7 string tuned down one full step to ADGCFAD
I am having difficulty figuring out how to play the riff from the first ~20 seconds, any help would be appreciated.

The song is playing in this video: http://www.youtube.com/watch?v=b0qRPHk1S_8