Well if it's supposed to follow the way it would have been said in Nordic countries around the time Iceland was being settled then it would be "Draugar", else if they're just using English spelling and grammar I take it would just be "Draugr"...
Actually, Draugar is how Icelandic people would say it back then, but Draugr is how their progenitors, the settlers of Iceland (from Norway) said it. Though, it's just about how you write it, as "Draugar" and "Draugr" are pronounced exactly the same way in Norse, Norwegian, and Icelandic.