Rebellion of Jang Bogo
When true-bone nobles blocked his daughter’s marriage to the king, Jang Bogo rebelled, and was assassinated by Yeomjang, an agent sent by the court.

Rheo1905 · CC BY-SA 3.0 · Source
In 839 Jang Bogo helped Kim U-jing, who had lost a succession struggle, overthrow King Minae and take the throne as King Sinmu. Sinmu’s son King Munseong agreed to marry Jang’s daughter, but the nobles objected to a queen from an island family; Jang then rose in revolt from Cheonghaejin.
The court sent Yeomjang to kill him, and in 851 abolished Cheonghaejin and moved its people to Byeokgol (Gimje). The Samguk sagi dates Jang’s death to 846; Japan’s Shoku Nihon kōki gives 841.