Complex sentence: As Crown Prince Nguyễn Phúc Cảnh had already died, it was assumed that Cảnh's son would succeed Gia Long as emperor, but, in 1816, Nguyễn Phúc Đảm, the son of Gia Long's second wife, was appointed instead.
Split into simpler sentences: As Crown Prince Nguyễn Phúc Cảnh had already died, it was assumed that Cảnh's son would succeed Gia Long as emperor. Thus, in 1816, Nguyễn Phúc Đảm, the son of Gia Long's second wife, was appointed instead.