For Masamune in the vanilla IVAN, see E-numa sa-am.
"Your Masamune's slash cuts the <monster> so deep you thought it was cut in half for a moment."
Like Muramasa, Masamune delivers striking damage instead of slashing damage due to a scripting oversight.
Masamune is an ancient sword named after Masamune a legendary swordsmith widely considered to be the best in Japanese history. Masamune's blades had a reputation of superior beauty and quality not commonly found in an era where quality steel was a rarity.
Legend tells of a competition between Muramasa and his master, Masamune to see who could produce the finest sword.
Variations of the tale exist but the general story remains the same - Muramasa and Masamune produce their masterpieces and test them by placing them in a river. Muramasa's sword slices anything it touches cleanly in half - including fish, leaves, and even the air blowing past it. Masamune's sword leaves everything in the river untouched, and fish even approach it out of curiosity - in some versions of the tale it even sews Muramasa's victims back together. Depending on the tale, a third party intervenes to judge, ruling that Muramasa's sword - while of unquestionable quality - is evil and bloodthirsty, whereas Masamune's sword is holy and just for not causing unnecessary harm.
However, Muramasa lived a full 300 years before Masamune, so it is impossible this encounter ever happened.