Dayun held a position within the Imperial Navy's officer corps throughout the Galactic Civil War. As an admiral, he was in command of the Star Destroyer called the Implacable. Not long after the Battle of Yavin in 0 ABY, Dayun told Lieutenant Commander Del Meeko, the ship's chief engineer, that he was being reassigned at the request of Admiral Garrick Versio, who specifically asked for the engineer.
The admiral's face, usually ruddy, was noticeably red during this discussion, and he addressed Meeko by his given name rather than his rank, indicating the degree to which the information had unsettled the officer. Following this, Meeko left the Implacable, and Versio brought the commander into Inferno Squad, the Imperial Special Forces unit.
The 2017 novel Battlefront II: Inferno Squad, written by Christie Golden, included a mention of Dayun.