“A powerful blade, and regalia of House Archanea.”
—In-game description, Shadow Dragon
Mercurius (ミラクルソード Mirakuru sōdo, lit. Mercury Sword; Miracle Sword in fan patches) is a legendary Sword that appears in the Akaneia Series of the Fire Emblem franchise, alongside Fire Emblem: Awakening. One of three Regalia weapons of Archanea, this sword serves as the counterpart to the Gradivus lance and the Parthia bow. Taking its roots from an unidentified origin, this sword was purloined by the enigmatic thief Adrah, where it played a pivotal role in his crusade of conquering the continent of Archanea. Upon the formation of the Kingdom of Archanea, Mercurius was bequeathed the honorific of being one of three hallowed Regalia weapons, and subsequently passed down through generations of Archanean royalty.
During the War of Darkness that occurs in Ankoku Ryū to Hikari no Tsurugi, Book 1 of Mystery of the Emblem and their remakes, this sword is stolen by the invading Grustian troops. Est, upon learning of the vexing fact, then embarks on a mission to recover it, and thereafter passes it into the possession of Marth's army after rejoining it. The sword is then returned to the vaults of House Archanea, where it is ceded upon Astram when he assumes the role of the captain of the newly-assembled Archanean Free Company.
During the War of Heroes that takes place in the sequel and its remake, the Mercurius blade is observed to be the primary weapon wielded by Astram in Chapter 16, where he is convinced by Jeorge to defect to Marth's army, in accordance to his loyalty to Nyna and his desire to live for Midia's sake.
In Ankoku Ryū to Hikari no Tsurugi, the Mercurius is referred to as the Mercury Rapier by Nyna at the end of Chapter 12, that which may explain why it is only wieldable by Marth. This is, however, rectified in the later games, where the sword can be equipped by any sword-wielder.
The sword's name is derived from Saint Mercurius, a Christian martyr whose name essentially carries the meaning "wielder of two swords". According to historical scriptures, the second sword that he wielded was a gift bequeathed upon him by the Archangel Michael.