Masamune
The Masamune (グランドリオン, Grandleon) is a weapon used in Chrono Trigger and Chrono Cross.
Chrono Trigger
In Chrono Trigger, the Masamune acts as a weapon of Frog's, gained once Frog joins the party permanently. In a flashback, Cyrus also wields it himself in order to stop Magus, only to fail. Masa & Mune make a reference to Cyrus being the last person before Crono and his party to make it to them. It can be charged later in the game in The Hero's Grave sidequest to be even more powerful and become Frog's most powerful weapon. When used upon Magus, it can lower Magus' defense massively to standard attacks. One half of the blade resides with Frog's ownership and the other half is guarded by the boss of Denadoro Mountains.
Chrono Cross
In Chrono Cross, the Masamune is a blade of pure evil. It drives those who wield it insane as long as they hold it. Radius has connections with by the fact that he and Garai went to the Divine Dragon Falls. There inside the cave atop the falls they found the Masamune. As they were leaving however, the maddness that comes with the sword got to Radius and caused him to kill Garai against his own will. Dario has the greatest connections with the blade in this game though. One day Dario and Karsh went the Isle Of The Damned in search of the Masamune. When they found it, Dario in joy of finding it grabs the sword without thinking of the consequences and is driven to kill Karsh. Karsh is forced to kill Dario. Lynx shows not too long afterward and convinces Karsh to tell everyone demons killed Dario. This later comes back to haunt all connected to the blade and Dario. By going to the Forbidden Island in the Home World, you can find Dario whom survived Karsh's attack in this world but in return he 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 achieves the player ownership of the most powerful weapon in Chrono Cross, the Mastermune. The sword also acts as a plot device in Chrono Cross, blocking the team's way into the Dead Sea forcing them to fight a former ally in the Isle Of The Damned.