Louis VIII of France
William took the cross in 1209 and left Anjou to crusade in the Toulousain and Languedoc with the Eudes III of Burgundy against the Cathar heretics. He was active at the siege of Béziers in July and Carcassonne in August. He wouldn't again participate in the Albigensian Crusade until 1219 at the Siege of Marmande under Prince Louis (later Louis VIII of France, the eldest son and heir of King Philip II of France.