Complex sentence: In September 1820, at the age of 38, Mianning inherited the throne after his father the Jiaqing Emperor died suddenly of unknown causes, becoming the first Qing emperor who was the eldest legitimate son of his father.
Split into simpler sentences: In September 1820, at the age of 38, Mianning inherited the throne after the Jiaqing Emperor died suddenly of unknown causes. He became the first Qing emperor who was the eldest legitimate son of his father.