After Miyamoto Musashi had killed Yoshioka Denshichiro leadership of the Yoshioka house and sword school fell to the 12 year old Yoshioka Matashichiro.
Shortly after the demise of his brother, the new scion of house Yoshioka challenged Musashi to a duel to the death to reclaim the honor of his house and to avenge his brothers. In the terms of the duel it was requested that the duel take place outside of Kyoto and at night. Musashi suspected duplicity on the part of Matashichiro but agreed anyway.
Unlike his previous duels Musashi arrived very early for this duel and hid in the trees and waited. Matashichiro suspected that Musashi, as usual, would be late and arrived at dusk with a cohort of retainers.
Musashi watched at the Yoshioka retainers fanned out and hid themselves among the bushes while Matashichiro, dressed in full armor, stood in the clearing, waiting for Musashi to arrive.
When the time was right, Musashi leaped from his hiding spot and in one swift stroke decapitated the hapless Matashichiro. The Yoshioka retainers swarmed from their hiding places. Even though he was seriously outnumbered, the effect of seeing their plan disintegrate… their liege lord killed and fighting in the dark against an apparent demon, Musashi was able to hold them off and escape.
It is said that in this battle Musashi first used both his swords, using one to defend and the other to attack. A notion that was completely antithetical to the sword forms of the time and would later form the basis for the Nito-Ryu style of Kenjutsu.