Short answers, pulled from the story.
Masamune lived and worked between 1288 and 1328 during the twilight of the Kamakura period and the dawn of the Nanboku-chō era. No exact dates mark his birth or death, and the historical record offers only a vague window of activity spanning these years. He worked in Sagami Province, a region that would become the cradle of the Sōshū school.
Masamune introduced the notare hamon, a finish where the leading edge of the blade slowly undulates like a gentle wave. This structural triumph balanced the brittleness of the hard edge with the flexibility of the soft core. His works are characterized by striking chikei and kinsuji, which are dark lines following the grain pattern and lightning-shaped lines of nie that shimmer like frozen electricity.
The Honjō Masamune vanished into the fog of history shortly after the end of World War II. In January 1946, the Mejiro police gave the sword to a man identified as Sgt. Coldy Bimore of the Foreign Liquidations Commission, and from that moment, the blade disappeared. Despite decades of searching, its fate and current location remain unknown, making it the most important of the missing Japanese swords.
Masamune trained fifteen known swordsmiths, with ten considered to be the Juttetsu or Ten Famous Students. Among these disciples were Chogi, Kanemitsu, Shizu Saburo Kaneuji, Kinju, Kunishige, Kunitsugu, Saemonzaburo, Saeki Norishige, Go Yoshihiro, and Naotsuna. These disciples carried the torch of the Sōshū school, ensuring that Masamune's legacy would endure for centuries.
A legend tells of a test where Masamune's pupil, Muramasa, challenged his master to see who could make a finer sword. They suspended the blades in a small creek with the cutting edge facing against the current, and Masamune's sword cut only leaves while Muramasa's sword cut everything including fish and air. A monk explained that Masamune's sword was holy while Muramasa's was evil, establishing both smiths as symbols for their respective eras.
The Kyōhō Meibutsuchō lists 248 famous swords, among which 80 had already been lost during the Siege of Osaka and are included only as historical records. Masamune is represented by 59 swords, 18 of which had already been lost at the time the catalog was compiled, accounting for an overwhelming one quarter of the entire list. The catalogue also includes 9 of Masamune's swords designated as National Treasures and 10 as Important Cultural Properties.