According to Deadline, stunning British actress Melanie Liburd (The Grinder, Strike Back, Dracula, CSI: Crime Scene Investigation) has joined season 6 of HBO's Game Of Thrones as a character known only as The Red Priestess. Of course, we already have one Red Priestess: the pyromaniac cloths-hater known as Melisandre (Carice Van Houten).
Deadline suggests that this might be a "new incarnation" of that character, but it's far more likely that Liburd is simply playing another R'hllor-worshiper. Some leaked script details from a couple of months ago outlined a scene in which Tyrion and Varys converse with a "red woman", so unless that was supposed to be Melisandre and she wasn't named for some reason, mystery solved.
Game Of Thrones is currently shooting, and should return next April.