He was captured and imprisoned (as an act of retaliation) by Llywelyn the Great, ruler of most of Wales, in 1229, but was given relative freedom because he was related to the prince by marriage in several ways - his daughter, Isabella, was married to Llywelyn's only legitimate son. However, in the following year, Llywelyn is said to have discovered that his wife, Joan, had committed adultery with William, and had William hanged on May 2, 1230. Joan herself was later pardoned.