Gokuaku no Hana - Hokuto no Ken - Jagi Gaiden

One day, a wanderer appears out of the wasteland to bring justice to the guilty and hope to the people—a warrior named Ken, a man who bears seven scars upon his chest and holds the secret of a mysterious martial art known as Hokuto Shinken, the Divine Fist of the North Star!

(Source: VIZ Media)

A oneshot story about Kenshiro's sifu Ryuken and also the other adoptive brothers. Elaborates on how he chose the Hokuto Shinken successor and the events related to it.