From the festival's Facebook:
The bad news is that Onslaught is out. This is because their record label, AFM Records, wants to postpone their USA tour so they can promote their (Onslaught's) new album more in Europe. Unfortunately, this means that Exumer, Bonded by Blood, and Fueled by Fire are dropping out as well since they were a part of the original Onslaught tour over here in the USA.
9/11-13 Riot Fest
9/19 Uncle Acid & the Deadbeats
9/25 Cattle Decapitation
9/26 D.R.I. and/or Alcest