The Masamune Katana is an incredibly good weapon. It deals almost as much damage as a sledge, but attacks much quicker and has a better range. The description says, "The greatest sword in the history of... swords."
How to Unlock/Earn
You receive this by maxing out the job of Video Game Designer. It is a reference to Final Fantasy, since there is a sword called Masamune in many Final Fantasy games, for example Final Fantasy VII, as Masamune is the name of Sephiroth's sword. It could also be a reference to Chrono Trigger, as it was the legendary sword usable by Frog.
Upon earning the sword, the game will display a message reading: "Congratulations! You reached max rank as a Video Game Developer. Miyamoto 2.0"
The name of the maxed out title is a reference to Miyamoto Shigeru, main developer of the Super Mario series.