Unit 684

209th Detachment, 2325th Group
Active1 April 1968 – 23 August 1971
Country South Korea
Branch Republic of Korea Air Force
TypeSpecial forces
RoleBlack operation
Bomb disposal
Close-quarters battle
Direct action
HUMINT
Irregular warfare
Long-range penetration
Manhunt
Special operations
Special reconnaissance
Tracking
Size31
Part of20th Special Service Squadron
Garrison/HQSilmido
Nickname(s)Unit 684 (Korean: 684부대), Silmido Unit

209th Detachment, 2325th Group (Korean: 2325부대 209파견대), commonly known as Unit 684 (684부대), was a special forces unit of the Republic of Korea Air Force (ROKAF) that specialized in black operation, direct action, irregular warfare, long-range penetration, and special operations that are extremely high-risk and dangerous. It was formed to assassinate North Korean leader Kim Il Sung in 1968, in retaliation for the North's botched Blue House raid.

The unit consisted of 31 civilian recruits, mostly petty criminals and unemployed youths, and underwent three years of harsh training on the island of Silmido. The assassination mission was cancelled in 1971 and the unit mutinied, resulting in a firefight in Seoul in which most of the members of the unit were killed. The four survivors were sentenced to death by a military tribunal and executed.