Secret of the Ninja is book 16 of the modern Choose Your Own Adventure series. It was written by Jay Leibold.
On a visit to your friend Nada's dojo in Japan, strange forces seem to be present. Winds howl on calm, clear days, lightning appears out of nowhere, freakish sounds are heard at night. Is it coincidence, or the arrival of a mysterious samurai sword sent by an anonymous donor? Nada thinks the sword is cursed, but who ever heard of a sword possessed by a demon? You agree to undergo a trance to help find out, with devastating results. Should you and Nada go back in time to feudal Japan to find the sword's owner and solve the mystery for good? Can you learn the ways of the ninja needed to do it? If you leave, will you ever return? Or will you call into the grasp of the dreaded Sanchiro Miyamotori?