The Sword of Seals was originally wielded by the hero Hartmut alongside the Eckesachs and used to fight against the Manaketes during The Scouring. During the final stages of the war, Hartmut and the other Heroes attack the innermost part of the Dragon Temple. There, Hartmut finds the Mage DragonIdenn in her human form, staring out into space and barely noticing their presence. Despite his own hesitation, Hartmut is forced to defend himself after she transforms and retaliates in response to the attack of the Eight Heroes. As if feeling the pity that Hartmut felt towards Idenn, the final blow from the Sword of Seals instead sends her into a state of slumber. Using both the sword and the Fire Emblem, Hartmut then proceeds to seal Idenn in the Dragon Temple, and later establishes the nation of Bern around it as a means to safeguard said seal from being broken. After the Scouring reaches its conclusion, the Sword of Seals is, alongside the DarkTomeApocalypse, sealed within the depths of the Shrine of Seals.
Later, in the events of Binding Blade, the seal imposed on the Sword of Seals is released by Roy, whereupon he lifts it from its resting place in order to aid him in his subsequent battles against Zephiel and the reawakened Idenn.
The blade is one of the most powerful weapons in the series, stronger even than the Legendary Weapons of Elibe. Roy is able to use ranged attacks with the blade, and even gains a new, more acrobatic fighting style with it. He also gains a new critical animation where the sword blazes with flames as he strikes the enemy.
When Roy uses the Sword of Seals, the ranged critical attack animation is very similar to the normal ranged attack animation, but he holds the sword up slightly longer. This most likely comes from reuse of the melee critical animation, which is similar to the ranged animation in itself.