The Mithril Sword is a battle item that appears in Riviera: The Promised Land. This blade can be crafted from the Mithril ore found on Tetyth's Aura Plateaus, but also drops from defeated enemies. After completing Chapter 4, return to Chappi with the Mithril in tow and he will craft one (x50) in exchange for all swords; he gives the Holy Sabre instead if all rapiers are traded. A chest containing 50x Mithril Sword appears in Yggdrasil (Chapter 6) as well.
Late in Yggdrasil, a chest can be opened to reveal the fairy KiKi, who can bless it to transform it into an equal amount of Zephyr.
In the GBA version, the Mithril Sword can invoke a glitch that lets the player use the enemy-exclusive DRK element. This is easiest to see in the practice battles in Yggdrasil that contain the Warrior-type enemies (who have Previous Element in all their close-range attacks). If the player uses a holy attack against them, they will counter with Dark. If the character who was hit then uses a Previous Element skill to copy the copied Dark attack, the character will use Dark to attack. The icons for Dark even appear in the skill window, and the animations also change accordingly (e.g. the player will see the 'pillar of darkness' appear with Fia's Rising Edge). The damage formula behaves strangely as well: the damage can range from normal to very poor. This is the only way to use Dark-type attacks without a cheating device. Similarly, this glitch also allows enemies to use copied Holy-type attacks against the party, with some attacks even gaining the ability to inflict Purify.