Seed Thralls are mission bosses from Final Fantasy XI: A Crystalline Prophecy - Ode of Life Bestowing. They are summoned by the Seed Crystal in the battlefield Ode of Life Bestowing.
One Seed Thrall is summoned whenever Seed Crystal uses the move Seed of Deception. This move creates a Seed Thrall who looks identical to one of the players in the battlefield. Up to six Seed Thralls can be spawned at a time in this battlefield.
Seed Thralls attack normally and are also capable of using weapon skills based on what weapon was copied as well. These include weapon skills that the players themselves may not have access to. Seed Thralls copied from Rangers will use ranged attacks instead. Seed Thralls also inherit weapon properties from the players they are copied from. As an example, a Seed Thrall copied from a player wielding the Ice Staff will have the occasional additional effect of ice damage.