An american tries to steal a samurai sword from some kind of compound. He stops and realizes if he hadn't they would have killed him. He joithe master, after having to prove himself (they bury him in the ground with only his head exposed). They then teach him how to use the samurai sword and he helps to retrieve a special swords twin. They are two important family swords and one was taken and the remaining master vows to return the two tegether.

I found it, it's called "The Challenge" with scott Glen made in 1982.