Complex sentence: Narnia was then ruled by the Telmarine King Miraz, who had murdered his brother Caspian IX and usurped the throne, and who planned to murder the true heir, his nephew Prince Caspian after the birth of his unnamed own son.
Split into simpler sentences: Narnia was then ruled by the Telmarine King Miraz, who had murdered his brother Caspian IX and usurped the throne. Miraz planned to murder the true heir, his nephew Prince Caspian after the birth of his unnamed own son.