The most famous musha shugyo was that of Miyamoto Musashi. 500 years ago, as a thirteen-year-old he killed a man in a duel, and set out to see what he might become. At the end of his Journey he wrote A Book of Five Rings. Here are quotes, and an interpretation.
“The primary thing when you take a sword in your hands
is your intention to cut the enemy, whatever the means.