Surene priest

 was presiding over the burial ceremony of the Surene king in Lower Mesopotamia in the 11th century B.C. He made sure that Prim was buried alive in the tomb of her husband, as was a Surene tradition. (AUDIO: The Boundless Sea)

Behind the scenes
Aaron Neil is uncredited for this role but states in the CD Extras for the box set The Diary of River Song Series One that he plays a priest.