The Balmung (バルムンク Barumunku, translated Balmunc in the Fire Emblem Museum and Balmunk in Fire Emblem Treasure) is one of the many holy weapons in Fire Emblem: Genealogy of the Holy War. The Balmung is also the insignia for the Kingdom of Isaach.
It was first used by the Crusader Odo, one of the original Twelve Crusaders. Shanan is the only one who can have it, though technically anyone with full Odo blood can use it. What this is referring to is that if Larcei or Ulster can wield it if their father was Holyn with the help of a glitch.
While the sword is equipped, it grants 10 Skill and 20 Speed. It thus has the same stat boost as Forseti but with 2 less weight (it has 3 weight while Forseti has 5 weight).
After the Isaachian-Grannvaleian war, it is placed in the Yied Shrine. During Chapter 6, Patty infiltrates the Yied Shrine and snatches the Balmung; Shanan, who has gone to retrieve it, catches her doing so and convinces her to hand it to him instead, since he is the only one able to use it. Patty is doubtful at first, but upon acknowledging Shanan's identity, she cheerfully accepts since she has admired him for years.
Balmung must have been named after Siegfried's (Sigurd) sword in the Nibelungenlied. This is a translation from the Norse myth, where Balmung is known as the Gram (Meaning 'wrath' in Old Norse), which was used by Sigurd to kill the dragon Fafnir. It is called Nothung in Richard Wagner's Ring Cycle.
The sword was forged in the Wayland of Smith and originally belonged to Sigurd's father, Sigmund, who received it in the hall of Volsung after pulling it out of the tree Barnstokk into which Odin had stuck it, believing no one else could pull it out. The sword was destroyed and reforged at least once. Afterwards, it could cleave an anvil in twain.