The performance from the group, which is comprised of former Black Sabbath stars Tony Iommi, Geezer Butler, Vinny Appice and Ronnie James Dio, has been announced as a UK festival exclusive.
Other acts recently added to the event include Coheed & Cambria, The Ataris, F**ked Up, Hundred Reasons and Zebrahead.
Hundred Reasons' drummer Andy Bews said: "We're really stoked to be added to such a stellar lineup!
"Nothing beats people telling you they still remember the time you played a festival years and years ago - managing to stand out among a ton of great bands is really special. And with Metallica, NIN, Killing Joke and Anthrax at Knebworth, how can this not be amazing?"
Other bands booked for the weekender on August 1 and 2 include Machine Head, Killing Joke, Thin Lizzy and Linkin Park.