Shitsugo no Ken

The Dark Ages, also known as the "Hundred Empty Years," was a period of time in history for which there are no records. But legends say there was a power during that time that far exceeds anything from the current civilization. Today, Tokyo is a city of dojos. The master of one dojo, Hachikazu Soudou, a "one-versus-thousand" master of sorcery, is overcome by a sense of helplessness, as he could only hold the hand of his dying childhood friend. That was when a government agent, Shizuku Yabusaki, comes to him with a surprising proposal: "Let's go back to the Dark Ages together!" (Source: Bessatsu Shonen, translated)