Sejo, personal name Yi Yu, sometimes known as Grand Prince Suyang, was the seventh monarch of Joseon. He was the second son of Sejong the Great and the uncle of King Danjong, whom he forced to abdicate in 1455, after having previously led a coup d'état against him two years earlier. In his quest to seize power he ruthlessly killed rivals, including his younger brothers, Grand Prince Anpyeong and Grand Prince Geumseong. However, during his reign, he reformed administration, led campaigns against the Jurchens, revised land laws, promoted literature, and established court music. His Kyŏngguk Taejŏn became Joseon's foundational legal framework.