Complex sentence: Simon IV de Montfort was the leader of the Albigensian Crusade to destroy the Cathar heresy and incidentally to join the Languedoc to the crown of France.
Split into simpler sentences: Simon IV de Montfort was the leader of the Albigensian Crusade. The military response to the Cathar heresy, with the secondary intention of joining the Languedoc to the crown of France.