With the population of Westeros dwindling, HBO has announced that a crop of new characters will enter the fold in Season 5 of its hit drama “Game of Thrones.” The stage and screen veteran Jonathan Pryce and Keisha Castle-Hughes, a best actress Oscar nominee for the 2003 film “Whale Rider,” are among nine new cast members set to join “Thrones” in 2015. While Season 4 brought the demise of several main characters, the coming season will try to fill the void with the Martells of Dorne. Most of the new characters are relatives or associates of Oberyn Martell (Pedro Pascal), the playboy prince who met a particularly gruesome death while fighting on behalf of Tyrion Lannister (Peter Dinklage). Ms. Castle-Hughes is to portray Oberyn’s eldest bastard daughter, Obara Sand. She is to be joined by Alexander Siddig (“Star Trek: Deep Space Nine,” “Da Vinci’s Demons”) as Doran Martell, the ruler of Dorne and older brother of Oberyn, and Toby Sebastian as Doran’s son Trystane. New additions not of the Martell bloodline are to include Mr. Pryce as High Sparrow, a prominent figure in a religious order; Nell Tiger Free, who replaces Aimee Richardson as Myrcella Baratheon; and DeObia Oparei as Areo Hotah, captain of Doran’s guards. The series is expected to return in the spring.