Yashaken was a katana said to be crafted by master swordsmith Masamune. The sword was made of triple-forged steel with a helm-guard carved with a ki-rin and believed to be enchanted, undefeatable in battle while wielded.
The sword found its way into the hands of samurai called Nishimori in the 1630s. Following a series of battles, Nishimori became disillusioned with his shogun when ordered to execute 35,000 prisoners.
He went into exile and took Yashaken with him, living his finals days as a Buddhist priest in a monastery on the island of Kangetsujima. Following the advice of a priestess, his help was sought by the island's villagers after an attack by pirates.
The blade was used to train the villagers and the pirates' next attack was repelled. Instead of leaving, however, the pirates kidnapped the priestess and used her as bait to lure Nishimori to them unarmed inside a mountain cave. The priestess was freed but Nishimori was killed.
Yashaken was left to a shrine where it stayed following a prayer by the priestess for vengeance. The mountain erupted and buried the pirates along with the village and the shrine, trapping Yashaken until its discovery once more in the 1930s.