A ninja oath, recently found in a centuries-old document in Japan, reveals how an aspiring ninja was sworn to secrecy about 300 years ago.

The ninja who penned the vow used cursive calligraphy to make a pledge to never reveal what he was taught, on pain of his descendants suffering divine wrath for generations, Agence France Presse (AFP) reported.

Experts can only guess if the ninja, who signed the document “Inosuke Kizu,” kept that oath to the end of his life. But details in Kizu’s solemn vow reveal how serious ninjas were about guarding the secrets of their techniques and abilities, AFP said.

Researchers found the oath among 130 ancient documents that were recently donated by the Kizu family, which was once a ninja clan in the town of Iga, near Kyoto. The oath likely was sent to Kizu’s relatives after his death, Yoshiki Takao, an associate professor with the International Ninja Research Center at Mie University in Japan, told AFP.

In the vow, Kizu pledged his thanks to the mentor who trained him in “ninjutsu,” or “the way of the ninja.” The term does not indicate the practice of a specific martial art, according to the Ninja Museum of Igaryu in Iga, Japan. Rather, ninjutsu is a type of “warfare art” specializing in espionage and strategic military attacks.

more at livescience.com