Complex sentence: Three years later he was one of the principal figures in the Albigensian Crusade against the Cathars, in recognition of which service he was made Bishop of Carcassonne (1211) and is commemorated in the "Cistercian Menology".
Split into simpler sentences: Three years later he was one of the principal figures in the Albigensian Crusade, which fought against the Cathars. In recognition of his service he was made Bishop of Carcassonne (1211) and is commemorated in the "Cistercian Menology".