Sudden death due to illness. Because he had been frail and sick since childhood, he also suffered from epilepsy. He had an epileptic seizure while going out and died suddenly.
Li Hong (652-675), Emperor Xiaojing of the Tang Dynasty, was the fifth son of Emperor Gaozong Li Zhi of the Tang Dynasty and the eldest son of Wu Zetian. The first prince in the dynasty to be posthumously named emperor.
Li Hong was born in the palace in the third year of Yonghui (652). In the sixth year of Yonghui (655), he was granted the title of Dai Wang. In the first year of Xianqing (656), he was established as the crown prince. He was filial and benevolent when he was young, and he was considerate of the people's sentiments for the crown prince. He once wrote to Emperor Gaozong to amend the unreasonable laws, and interceded for his two convicted sisters. He was deeply favored by Emperor Gaozong.
In the second year of the Yuan Dynasty (675), Li Hong traveled to Luoyang with the emperor and his empress, and died suddenly in the Qiyun Hall of Hebi Palace. He was only 23 years old. The cause of death is quite controversial. Emperor Gaozong was deeply saddened by this. He made an exception and posthumously made Prince Li Hong the emperor, with the posthumous title of "Xiaojing", and was buried in Gongling Mausoleum with the rites of an emperor.
In the early years of Shenlong (705), Tang Zhongzong Li Xian went to the temple to honor the emperor and gave him the name Yizong. In the sixth year of Kaiyuan (718), Li Longji, Emperor Xuanzong of the Tang Dynasty, revoked the temple title of Yizong in accordance with the old etiquette, and reused the posthumous title of Filial Piety Emperor for worship.
Extended information:
Biography of the character
Gaozong Yuanzi
In the second year of Yonghui (651), Emperor Gaozong of the Tang Dynasty felt that Wu Zetian, who was a nun in Ye Temple, was summoned into the palace again and given the title of Zhaoyi. In the third year of Yonghui (652), Wu Zetian gave birth to her eldest son Li Hong in the palace, who was the fifth son of Emperor Gaozong. Since Empress Wang had no heirs, the crown prince at that time was Gaozong's eldest son, Li Zhong.
In the sixth year of Yonghui (655), Emperor Gaozong deposed Queen Wang and Concubine Xiao Shu as common people, and appointed Wu Zetian as queen. Li Hong thus became Emperor Gaozong’s Yuanzi (eldest son). In the same year, Li Hong and his younger brother Li Xian were promoted to princes, and Li Hong was appointed king. In the first year of Xianqing (656), Emperor Gaozong issued an edict to depose the crown prince Li Zhong and make the four-year-old acting king Li Hong the crown prince.
Reference materials: Li Hong_Baidu Encyclopedia