I don't know if this is due to the fact that the Mayhem website is shit, but they haven't mentioned either on the official site or the Facebook that Betraying The Martyrs is part of the festival. I hope I'm wrong, but I think they the replacement band.
8/29 LSD and the Search for God
9/9 Creepers/Tony Molina/Drag Majesty
10/25-26 Thou & The Body