Who Is Valya Harkonnen in 'Dune Prophecy'?

The first trailer for Dune: Prophecy is here, taking viewers to a whole new era of the universe created by Frank Herbert. The HBO series is set 10,000 years before the events of the original novel, showing the origins of the order that eventually became known as the Bene Gesserit. What took most people by surprise is that most of the trailer is actually narrated by a character named Valya Harkonnen (Emily Watson). There is also almost no mention of House Atreides in the video, apart from her naming Paul Atreides (Timothe Chalamet) when contextualizing when Prophecy takes place. So who is Valya, and what role does she play in the origins of the Bene Gesserit?