Dark Dragon Blade

The Dark Dragon Blade (魔刀 「黒龍丸」 Matou "Kokuryuumaru", Devil Sword, Dark Dragon) is a dark and ancient sword made from the bones of the Dark Dragon.

Past
After the Dark Dragon was slain by the Dragon Lineage, the Fiends set out to create a sinister counterpart to their weapon, the Dragon Sword. Using the dragon god's bones, they forged the Dark Dragon Blade, which held the power of the Dark Dragon and the souls of his slain spawn, Black Dragon. With it, the fiends unleashed massive devastation and fed its hunger for destruction, but some time later, the Dragon Lineage finally managed to get the Dark Dragon Blade. However they were without a means of destroying it and sealed it instead, so as to prevent it from falling into the hands of those who would seek to unleash its dark powers. What the Dragon Lineage didn't know was that the True Dragon Sword could destroy the Dark Dragon Blade, but as time passed, its legacy soon became myth. The sword became a ceremonial relic, passed on from generation to generation for safekeeping without true knowledge of its dark powers.

Ninja Gaiden
However the Archfiend Vigoor learned the location of the Dark Dragon Blade and wanted to use it to regain his power. He sent his Greater Fiend Doku to retrieve the sword, who launched a massive attack on the Hayabusa village. Though Ryu tried to stop him, he fought the Dragon Ninja off and returned to Tairon, capital of the Holy Vigoorian Empire, where he presented the Dark Dragon Blade to Vigoor. Ryu pursued Doku and as he fought his way through the forces Vigoor sent against him in his quest to retrieve the blade, it fed off the deaths to become increasingly more powerful. Upon tracking the Dark Dragon Blade to the Vigoorian Emperor (an avatar of Vigoor), Ryu had a vision of him unsealing the blade as the Dragon Ninja combined the Eye of the Dragon with the Dragon Sword, freeing the True Dragon Sword. He found his way to the core of the Zarkhan Palace and eventually the realm of Vigoor, where the Archfiend had the Dark Dragon Blade, having restored his powers with it. Ryu and Vigoor battled with the Dragon Ninja emerging the victor and set out to escape the cavern, finally succeeding with help from the fiend hunter Rachel. In order to have his hands free to climb out, Ryu threw the Dark Dragon Blade out of the pit, where it landed at the feet of the Dark Disciple and his minion, Gamov, who'd been tracking the sword as well. The Dark Disciple took up the Dark Dragon Blade and used it to slay Gamov, before revealing himself as Murai, Ryu's uncle and leader of the Shadow ninja Clan, who'd brought about the theft of the Dark Dragon Blade so as to increase its powers. He then broke the seal and transformed into the Devil Incarnate and fought Ryu, but the Dark Dragon Blade's power could not match the True Dragon Sword. The Dragon Ninja defeated him and Murai dropped the blade as he fell to his demise in the Emperor's lair. Ryu then kicked the Dark Dragon Blade into the air and shattered it with the True Dragon Sword, bringing an end to its evil power.

Powers and Abilities
Due to being made from the Dark Dragon, the Dark Dragon Blade is quite powerful and its power continually increases with every kill in the area near it. When unsealed, it can transform a person into a form known as the Devil Incarnate, an all powerful dark being.

Trivia

 * The Dark Dragon Blade's design is based on the Seven-Branched Sword, a sacred artifact from Japan.
 * In gameplay, the Dark Dragon Blade is treated as a heavy weapon since Ryu puts it on his back after a combo. But in the last level of Ninja Gaiden, he holds it with one hand, as if it were a lighter weapon.