Sima Yuanxian (司馬元顯) (382[1] – 20 or 22 April 402[2]), courtesy name Langjun (郎君),[3] formally Heir Apparent Zhong of Kuaiji (會稽忠世子), was briefly a regent of the Eastern Jin, during the reign of his developmentally-disabled cousin Emperor An.
Sima Daozi, in fear, panicked and spent his days drinking, entrusting all important matters to Sima Yuanxian, who at the time appeared intelligent and resolute, reminding his followers of Emperor Ming.
Sima Daozi, believing that his son had made the right predictions, trusted him even greater after this point.
While he appeared to have some abilities, he also was, according to traditional historians, wasting the government's money in luxury and not paying attention to the burdens of the people.
Late in 399, Sun En, seeing how the only province remaining under Jin imperial government's actual control—Yang Province (揚州, modern Zhejiang and southern Jiangsu) -- had been mismanaged by Sima Yuanxian, launched a major attack from Zhoushan Island, briefly taking over nearly all of Yang Province and advancing on the capital Jiankang (modern Nanjing, Jiangsu).
[6] Jiankang fell to Huan, and Sima Yuanxian was captured and executed, along with all of his major associates and six sons.