The Mystletainn (ミストルティン, Misutorutin, alternatively translated Mistolteen in Fire Emblem Treasure, alongside being known as Mistoltin, Mistilteinn, or Mistletain in certain fan translations) sword is one of twelve holy weapons from Fire Emblem: Genealogy of the Holy War, and is the blade that was once wielded by Crusader Hezul, bestowed upon him by an unnamed Divine Dragon during the Miracle of Darna. Mystletainn is the property of the descendants of Hezul, who are the heirs of House Nordion of Agustria. It is sometimes called the Demon Sword, though the reason for such a name is unknown.
In Genealogy of the Holy War, only individuals in possession of major Hezul holy blood are able to wield the blade, with the only two characters fulfilling this requirement being Eldigan and Ares.
In Genealogy of the Holy War, the Mystletainn makes its first appearance in Chapter 1, where Eldigan uses the blade as he and his Cross Knights guard Nordion Castle and oversee the Verdane-Agustrian border on Sigurd's behalf. The blade appears again in Chapter 3, where Eldigan is forced to wield it against Sigurd under the command of Chagall. After being persuaded by his half-sister Raquesis, Eldigan leaves the battlefield, and meets his demise at the hands of the king he has pledged loyalty to.
The Mystletainn later appears in Chapter 7, where it is revealed to have been inherited by Eldigan's son Ares, who is stationed at Darna with Leen/Laylea and working under Jabarro's mercenary band. Despite the paternal role that Jabarro had for him following the fall of House Nordion and the death of his mother, Ares turns against him after learning of his support for Bramsel's advances on Leen/Laylea. Using the Mystletainn, Ares successfully rescues Leen/Laylea and later joins Seliph's army after learning of the bond shared by their parents.
In the events of Awakening, Owain, while embarking on a quest to locate the Mystletainn blade of yore, chances upon the "Missiletainn" blade instead. Unaware that the Missiletain blade is but a poor replica of the one he seeks, Owain takes it into his possession and proclaims it as his own.
The name "Mystletainn" most likely establishes its roots from the sword that Höðr employed in order to kill Baldr. According to the 13th century text Prose Edda, as a result of the devious schemes concocted by Loki, the god Baldr was slain by the hand of his brother, the blind god Höðr, by means of a mistletoe projectile. This mortifying act took place in spite of the vain attempts of Baldr's mother, the goddess Frigg, to beseech all living things and inanimate objects alike to swear an oath, one in which they pledge to not rein harm upon Baldr after he experienced terrifying nightmares of his death. Frigg was, on the whole, able to convince one and all to observe the oath, save for the mistletoe, by virtue of an oversight on her part, where she perceived it as being "too young" to demand a pact from.
In the Gesta Danorum version of the tale, Baldr and Höðr were instead portrayed as rival suitors, with Höðr killing Baldr with a sword named Mistilteinn (Old Norse "mistletoe").
To further add on to the existence of the Mystletainn blade in lore, a sword by the same name also manifests within various other Norse legends.
According to a conversation that can be viewed when visiting a certain village during the events that occur during the second generation of Genealogy of the Holy War, it is asserted that the Tyrfing, Balmung, and Mystletainn are the strongest blades to have ever been forged.