Complex sentence: He succeeded his father Elias as head of the House of Bourbon-Parma upon his death in 1959, and maintained his style until his death in 1974 in Vienna.
Split into simpler sentences: He succeeded his father Elias as Duke of Parma upon his death in 1959. He continued to be styled as Robert II, Duke of Parma, until his death in 1974 in Vienna.