In Chrono Trigger, the Masamune acts as a weapon of Frog's, gained as an equipable item once the party travels to the Magic Cave after Frog joins the party permanently. In a flashback, Cyrus wields it to stop Magus, but fails. Masa & Mune make a reference to Cyrus being the last person to successfully scale the mountain and arrive at the resting place of the legendary blade. When used upon Magus, it can lower his defense massively to standard physical attacks. One half of the blade resides with Frog's Burrow and the other half is guarded by Masa & Mune atop Denadoro Mountains. After recovering both parts, the sword is taken to Melchior in Medina for repairs, since he was the blade's original creator. The swordsmith informs the party that Dreamstone is needed to resurrect the sword and bring it to its former glory. Traveling to Prehistory, a specimen of the precious stone is received from the chief of a small village. Once the sword is repaired, it is presented to Frog.
During the events of the activation of the Mammon Machine, it was revealed that the Ruby Knife is indeed the Masamune in the past. During a battle with the Mammon Machine, the Masamune is the only weapon that does not increase its attack nor defense once struck, but instead absorbs it for healing purposes. Equipping the Masamune along with the Hero's Badge greatly increases the blade's critical hit ratio.
After Dalton modifies the Epoch, the sword can be upgraded in the Hero's Grave, as part of a sidequest. If Magus is present, he will shield himself with his cape as the newly upgraded Masamune shines powerfully in mid-air.
The Fall of Guardia
In Chrono Cross, the Masamune is a blade of pure evil. It drives those who wield it insane for as long as they hold it. Radius came into contact with the sword when he and Garai went to the Divine Dragon Falls. There, inside the cave atop the falls, they found the Masamune. As they were leaving, the madness inside the sword got to Radius and caused him to kill Garai. Dario also has a significant connection to the blade. One day Dario and Karsh went to the Isle Of The Damned in search of the Masamune. When they found it, an overjoyed Dario grabbed the sword without considering the consequences and is driven to attack Karsh. Karsh is forced to kill Dario in self-defense. Lynx shows up not too long afterward and convinces Karsh to tell everyone demons killed Dario. This lie later comes back to haunt Karsh. By going to the Forbidden Island in the Home World, you can find a version of Dario who survived Karsh's attack, but in return has lost his memories. Putting Riddel in your party and talking to him will initiate the Masamune to appear and convince him to slay the team. Karsh and Glenn are the only other characters to have phrases unique to the situation.
Defeating Dario grants the player ownership of the most powerful weapon in Chrono Cross: the Mastermune.