These events led to Owain formally assuming his ancestral title of Prince of Powys on 16 September 1400 at his Glyndyfrdwy estate. With a small band of followers which included his eldest son, his brothers-in-law, and the Bishop of St Asaph in the town of Corwen, possibly in the church of SS Mael & Sulien, he launched an assault on Lord Grey's territories. After a number of initial confrontations between King Henry IV and Owain's followers in September and October 1400, the revolt began to spread. Much of northern and central Wales went over to Owain. Henry IV appointed Henry Percy – the famous "Hotspur" – to bring the country to order. Hotspur issued an amnesty in March which applied to all rebels with the exception of Owain and his cousins, Rhys ap Tudur and Gwilym ap Tudur, sons of Tudur ap Gronw (forefather of King Henry VII of England). Both the Tudurs were pardoned after their capture of Edward I's great castle at Conwy. In June, Owain scored his first major victory in the field at Mynydd Hyddgen on Pumlumon. Retaliation by Henry IV on the Strata Florida Abbey followed, but eventually led to Henry's retreat.