Master Seals (マスタープルフ Masutā purufu, translated as Master Proofs) are promotional items used throughout the Fire Emblem series. They are used to promote first-tier, non-Lord characters of Level 10 and above (except for Fire Emblem: Awakening, where it promotes all first-tier characters of Level 10 or more).
Master Seals are relatively rare. In Fire Emblem: Rekka no Ken, the item is known as the Earth Seal (地の刻印 Chi no Kokuin), in contrast to the Heaven Seal, which is used to promote Lords.
Additionally, the item will not promote characters of the Pirate or Thief classes. The Ocean Seal and Fell Contract are needed for these classes, respectively. The Master Seal works on these classes in the more recent games, however.
These items are more useful in the games which only use the promotional item system. In the games where promotion is an automatic occurrence (Fire Emblem: Path of Radiance, Fire Emblem: Radiant Dawn), promotion items are only used for promoting earlier than level 20. Since early promotion leads to lower maximum stats, it is not very useful in the long term.
Location(s): Chapters 23E/24H [Southeastern Village (E)/Central Village (H)], 26E/28H [Enemy Druid (steal in hard mode only)], 30E/32H (Start of chapter and Secret Shop)