The Halcyon was constructed sometime before 2010; her battle record prior to the Circum-Pacific War, however, remains unknown. At some point, she was assigned to an Osean Maritime Defense Force battle group under the leadership of the OFS Barbet, which was stationed in the Ceres Ocean.
On December 29, 2010, the Halcyon and her accompanying fleet detected a large number of Yuktobanian warships on radar. It soon became clear that the Yuktobanian armada had engaged a much smaller Osean fleet, which included the OFS Kestrel. As the Halcyon and her fleet approached, it began eavesdropping on the Kestrel's communications and soon discovered that the carrier had joined forces with several Yuktobanian warships. At approximately 1721hrs, the Halcyon received a request for assistance from the Kestrel.
However, the Halcyon and her fleet—whose nationalist sympathies remained unwavering—deemed the Kestrel and its fleet to be traitors for allying with the enemy, and proceeded to open fire on their countrymen. The Halcyon, along with her fellow nationalist warships, was subsequently sunk by Razgriz Squadron.