Dual Blade is the most legendary and powerful weapon in the Lufia series. Its origins are not known, but judging by the opening sequence of Lufia II: Rise of the Sinistrals, it may have been created by a Sinistral (possibly even Arek the Absolute). It originally served as a warning system to alert the Sinistrals of the mortal(s) strong enough to challenge their rule. However, Dual Blade later sided with Maxim and his descendants, The Hero from Lufia & the Fortress of Doom and Wain from Lufia: The Legend Returns.
In addition to being a mighty weapon, Dual Blade has a vast array of magical powers. It helped Maxim and his descendants to defeat the Sinistrals more easily by allowing them to channel their energy waves into itself and then increasing the output to cause devastating damage. The sword seems to be sentient and can "communicate" with a high-pitched ringing, as well as warp itself and appear before anyone it deems worthy. In Lufia II: The Rise of The Sinistrals, it appeared to Maxim one last time to help him destroy the last flotation stone in order to bring down the Fortress of Doom. In Lufia & the Fortress of Doom it even had the power to repair itself when approached by the hero after being broken by the Sinistral Daos.
Dual Blade's appearance varies among the games in which it appears.
During the prologue, Maxim is appearing wielding a twin sword, which is implied to be a form of Dual Blade (as the name suggests). After the battle against Daos, Maxim sacrifices himself by staying beside his fallen comrade and wife, Selan, as Doom Island sinks into the ocean. The Dual Blade is then laid to rest under the sea along with its master.
During the journey of Maxim's descendant, however, the Dual Blade is rediscovered under the sunken Doom Island and appears to take shape of a single-handed knight sword with a golden handle. During the battle against the Sinistrals and the brainwashed Lufia, The Hero unwillingly uses the blade to finish off his childhood friend, but instead of killing her, Dual Blade provokes a side effect which erases the parts of Erim, the Sinistral of Death inside her body, and causes her to lose her memory as she falls from a great height when Doom Island sinks into the ocean again. Dual Blade's whereabouts at the end of the game are unknown, but it's implied that The Hero leaves it on Doom Island, where it is then found by Wain many years later.
Dual Blade is first shown in the game intro, where Arek the Absolute poses a question about whether the Sinistrals or humans will rule the world before he summons the blade to himself.
In the game, Dual Blade is described to be a sacred blade treasured by the mermaid race. Maxim finds the sword resting at the mermaid's temple as he encounters Daos for the first time and engages into a battle of wills against him. Dual Blade chooses the hero, who then becomes its rightful owner.
During the final battle against the Sinistrals, the game expands on the original game's conclusion. Daos uses the last bit of his power to drop Doom Island into Parcelyte, but fortunately, with Erim's help, Maxim gains access to the core of Doom Island, destroying Spiritual Stones by channeling his power through the Dual Blade. After Maxim seals the final stone and averts a catastrophe, he dies, and the Dual Blade sinks along with the island.
In the Nintendo DS remake of Lufia II: Rise of the Sinistrals, while playing a second time, the small, black, jagged structure of the sword transforms into a spiky, golden one, when Erim appears just as Maxim and Selan are about to destroy the last energy core of Doom Island. It is a result of Maxim's increasing energy waves, which allow him to destroy Erim and unintentionally change the path of Doom Island, thereby saving his and Selan's lives.
When Dual Blade is transformed, it shoots golden arcs at Erim, who is attacking from a distance. Using Time Brake, a sort of a cross-like figure can be formed, which deals heavy damage to Erim.