Complex sentence: Las Cases and his son accompanied the ex-emperor to Saint Helena and acted informally but very assiduously as his secretary, taking down numerous notes of his conversations, which thereafter took form in the famous "Mémorial de Sainte-Hélène".
Split into simpler sentences: Las Cases and his son accompanied the ex-emperor to Saint Helena. He then acted informally but very assiduously as his secretary, taking down numerous notes of his conversations, which thereafter took form in the famous "Mémorial de Sainte-Hélène".