Complex sentence: Then together they entered Doriath and stood before the throne of Lúthien's father, who when he heard their story was amazed and thought Beren to be above all other mortal men.
Split into simpler sentences: Then together they entered Doriath and stood before the throne of Lúthien's father. When he heard their story was amazed and thought Beren to be above all other mortal men.