Ys IX Prequel: The Lost Sword (イースIX 前日譚 ～亡失の竜剣～ Ys 9 Prequel: The Lost Dragon Sword) is a short novel included in the limited editions of Ys IX: Monstrum Nox. It takes place between Ys SEVEN and Ys IX, bridging the timeline blank between the two titles.
A digital version of the novel was also included in the Steam version as a launch week special for buying the Deluxe or Ultimate editions in the first week after release.
The novel features Adol, Dogi, and a female blacksmith called Cesilica Kentia, who mentions Balduq to Adol and piques his interest. The novel's title The Lost Sword refers to Adol's ultimate sword from Ys SEVEN: Caliocerion.
The novel also makes minor references to the events of Ys VIII: Lacrimosa of Dana and the Primordials.