The Masamune also appears in Final Fantasy Legend II/SaGa 2. It is one of the 78 Magi. Although I don't remember exactly where it is obtained, I do know that the game's villain, Apollo, uses it on you during your fight with him. Later on, when you get Isis as a party member, she is able to use it in battle. The Masamune is a unique weapon in that it, along with the Xcalibur, are the only two weapons in the game that don't eventually break after a set number of uses.
The other mythological weapon in that game was the Gungnir, of course, used by Odin. I thought the game did an incredible job staying true to the gods. They even gave Odin his famous crows and his many-legged horse Sleipnir.
