 After 6 months on hiatus, the group returned with their 2nd mini album 'Eternal 0' and 'Eternal 1' in December 2016, thus completing the "Eternal" series.In February 2017, Hyung-jun began his mandatory military enlistment and is expected to return in 2019.On June 8, Double S 301 returned with their single, Unison Volume 1, to mark SS501's 12th anniversary.