Masamune is a legendary samurai warrior, with a conquest battlefield being named after him. He can be purchased in the Onyx Market, but is also available as a permanent bonus troop via the various YaoSquad statues or a Yao Memorial Statue.
The YaoSquad statues were available in the Legends of War: Yaosquad event and provide a Masamune only on attack. The Yao Memorial Statue is awarded to players who defeated Yao in the Apocalypse Yao Personal Tournaments and provide a Masamune on attack and defense.
He has a very low attack rate, so while he does a lot of damage, it's not a particularly high damage per second.