a French Cistercian at the convent of Fontfroide, near Narbonne, was commissioned as legate by Innocent III with two other monks of his order, Raoul and Arnaud, to combat with fire and sword the progress of the Albigenses. Castelnau showed great determination, but did not succeed in his purpose. Finally, having rebuked Raymond VI for his weak faith, he fled from court, but was overtaken near the Rhone and slain, Jan. 15, 1208. See Hoefer, Nouv. Biog. G neral, s.v.