The Icelandic singer will headline the summer bash at Knebworth Park, Stevenage on July 5.
Also playing the event will be Pendulum, Foals, Late of the Pier, Killa Kela, Battles, James Zabiela, Annie Mac and Plump DJs.
The festival runs for 18 hours between 1pm and 6am. Alongside the music there will be an Anglo-centric Victorian fun fair, a Twisted Tea Room and an enchanted wood.
Early bird tickets for the event are currently on sale.