The sacred sword Marth seeks in Shadow Dragon and the Blade of Light is Falchion.
Falchion is a divine weapon central to the game’s history. Long before Marth’s campaign, the hero Anri used it to defeat the Shadow Dragon Medeus. When Medeus returns, the sword is stolen after Marth’s father, King Cornelius, is killed. Marth must eventually recover Falchion and use it against the enemies threatening Archanea.
The sword is sometimes confused with other legendary weapons from the series. Tyrfing belongs primarily to the Jugdral games, Ragnell is associated with Ike in the Tellius games, and the Levin Sword is a separate weapon that uses lightning-based attacks. Falchion also appears in later games, including Awakening, where it remains strongly connected to Marth’s legacy.