Hierophants are the male leaders of druid circles. The female equivalents are called "flaminikas". The leader of the circle at Caed Myrkvid was a flaminique. It was her idea to take the old wives' tales of druids burning trespassers alive in wicker hags and make it a reality.
The Hierophant can be found in the Druids' grove in the swamp with the other druids once Chapter III has begun. He is a professional poker player and knows one of Geralt's old acquaintances, Mousesack (also a druid). Geralt, having amnesia, can not remember Mousesack, though.