The Fire Emblem is the imperial seal of Bern. During The Scouring, the hero Hartmut used the Fire Emblem and the Binding Blade to seal the Demon Dragon Idunn, and afterwards sealed the sword so that the seal needed to be placed in it before it could be drawn.
In Fire Emblem: The Blazing Blade the seal was needed for Prince Zephiel to be legally declared the heir to the throne of Bern, but it was stolen by the Black Fang under orders from King Desmond before the ceremony. Zephiel's mother, Queen Hellene orders Eliwood, Hector, and Lyn to retrieve the Fire Emblem from the Black Fang before she can show them the location of the Shrine of Seals.
In Fire Emblem: The Binding Blade, after ascending to the throne of Bern, Zephiel uses the Fire Emblem to free Idunn from her imprisonment in the Dragon Temple and begins his invasion of the other nations of Elibe. Zephiel's half-sister, Princess Guinivere, then stole the Emblem and fled into Lycia, running into Roy's army and later Saul, revealing that Zephiel did not want the Emblem falling into enemy hands and that she had hoped its disappearance would halt his conquest. Later, Guinivere and the Fire Emblem are captured by Narcian and brought before Zephiel who orders Guinivere's guard Melady to take her back to Bern and return the seal to the Shrine. Guinivere instead convinces Melady to join Roy's forces and gives the Fire Emblem to him. Later, Roy uses the Emblem to unseal the Binding Blade.