The Knights of Seiros (セイロス騎士団 Seirosu-kishidan) are a military organization in Fire Emblem: Three Houses.
Within the order are distinguishing ranks. There are squires, who train to become knights, the knights, who serve as the organization core forces, and the holy knights, the best warriors of the order, who can wield a Hero's Relic.
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A damaged book in the Shadow Library of Abyss titled "Burnt Remnants of a Report" mentions a clandestine group within the Knights of Seiros called the "shadowed order."