Seishitsu

Seishitsu is a katana forged and used during the Hyakunen Wars by Kanematsu Oda, Daimyo of the Oda Clan and the first emperor of what became a unified Yamatai. Upon the official end of the wars, the sword was hidden away until late-1542, when it came into the possession of Suguha Oda, First Princess of the Empire.

History
Seishitsu was first forged by Kanematsu Oda, a young blacksmith belonging to a lesser branch of Oda Clan, ten years after the onset of the Hyakunen Wars. Upset by all the battle and bloodshed that was going on, Kanematsu put his heart and soul into making an indestructible blade. Once the blade was finished, he used ancient Yamato magic to instill it with the powers of the elements.

Armed with the blade, Kanematsu demonstrated its power to the locals and rallied them to form an army. With the katana in hand and his followers behind him, Kanematsu restored unity to the divided Oda Clan, becoming Clan Leader in the process. With order restored, Kanematsu and his followers set out to unite Yamatai under one banner for the first time. Kanematsu eventually united the rival clans of his home island through diplomacy and conquest, displaying near genius military tactics and diplomatic charisma, thus founding the Yamato Empire. Over the following years he did same with the clans of the neighboring islands, expanding Oda rule far beyond his ancestral lands.

Fifty years after beginning his campaign, Kanematsu passed away and Seishitsu passed onto his grandson Akishige Oda, who proceeded to finish his grandfather's work in uniting Yamatai, but didn't manage to make much progress before dying of disease five years later. Upon Akeshige's death, Seishitsu then passed to Akeshige's nephew Chikayoshi. Chikayoshi then spent the next thirty years finishing the work of his grandfather and uncle before finally claiming victory. However five more years of rebellion against Oda rule ensued before peace became a reality.

Shortly before his death, Chikayoshi placed a charm on Seishitsu's sheath to prevent it from being drawn again until only the one most worthy gained it's possession. Chikayoshi then buried Seishitsu at the base of a Sakura tree behind what would become the Eimin no ji Shrine, the final resting place of the ashes all of Yamatai's Emperors.

700 years later, in late-1542, Seishitsu came into the possession of Kanematsu's descendant, First Princess Suguha Oda, Lunaculus of Aquilonis, and it has been in her possession ever since.

Powers & Abilities
Seishitsu's Powers revolve around the elements, Wind, Water, Earth, and Fire, hence it can only be wielded properly in the hands of a Bender.

Seishitsu is also indestructible, capable of taking any damage done to it and withstanding the passage of time. So even by the time Suguha came into possession of it, the blade never dulled.

Trivia

 * "Seishitsu", spelled "性質" in Japanese, is one of several words for "Nature" in Japanese.