In early medieval Japan, Onmyōdō or "the way of yin and yang" was a complex system of natural science, astronomy, divination, and magic that had its own bureau in the Imperial Court, consisting of official mystics known as Onmyōji.

The most famous of such yin-and-yang practitioners was Abe no Seimei (921-1005), a prominent historical figure also known as the greatest sorcerer in Japanese folklore and popular culture. A son of a powerful nine-tailed fox according to the myriad of legends, he grew up to be a detective-wizard-exorcist with accurate prophetic abilities and a dozen shikigami daemons under his command.

While most of Seimei legends take place in the Kansai region, which encompasses Kyoto and Osaka, some are known in Tokyo and its environs as well. There are a few in the North Kamakura neighborhood (though the lack of historical records suggests that he has never set foot in the area), and one of them has a rather unusual superstition surrounding it.

The so-called Seimei Stone, which has been placed on the grounds of Yakumo Shrine, once sat at the foot of a nearby bridge. Legend has it that stepping on it will grant you healthy legs, but there is a catch: you need to step on it unaware of this superstition, or else bad luck will fall upon you (so if you're reading this...don't dare do it).

The Stone was relocated to the shrine in the 1950s during the expansion of the road and has since sat there all but forgotten, making it almost impossible to step on it unknowingly.