A historical novel published privately by a distant descendant of the Koka ninja clan has proved to be an unexpected hit because it is rooted in fact.

In Japan, it is rare for a novel penned by an amateur to enjoy steady sales two and a half years after its release.

Titled “Ieyasu and the Koga Ninja Ohara Clan,” the book was authored by a resident of Shiga Prefecture under the pen name of Koga Otomohara.

The writer’s former position as an executive of a major corporation compels him to resort to the pseudonym when writing and speaking about the furtive feudal-era assassins.

A local samurai group has long been known as Koka or Koga. Many ninja families were formed within the clan, primarily through blood ties.

Otomohara claimed lineage from the Ohara family, one of the largest Koka ninja groups, which was famed for forging strong bonds among its members.

His book hit the market in December 2022. It combines episodes passed down orally by the Ohara family along with stories Otomohara collected through interviews with relatives and others.

The novel opens with a scene in which 18 elite members of the Ohara family place their hands in front of themselves and chant the famed spell: “rin, pyo, to, sha, kai, jin, retsu, zai, zen.”

The incantation is illustrated as “a ritual performed by Koka ninja before battle to pray to the sun and the moon.”

In October 1487, Shogun Ashikaga Yoshihisa (1465-1489) of the Muromachi Shogunate mounted a campaign to subjugate Sasaki Rokkaku Takayori (1462-1520), the military governor of Omi province in what is now Shiga Prefecture.

The Koka clan rallied to the Rokkaku side, staging a night attack on the shogunate’s war headquarters and ultimately defeating the enemy.

Of the 53 families fighting in the war, 21 related groups that distinguished themselves in battle received letters of appreciation from the Rokkaku clan.

The novel states: “The Koka clan has since been called the Koka 53 families or Koka 21 families, with its reputation for outstanding battlefield performance spreading throughout the country.”

The Ohara family is believed to have had especially close relationships with Tokugawa Ieyasu (1543-1616). The book is filled with anecdotes in this regard.

In the Honnoji Incident of June 1582, powerful warlord Oda Nobunaga (1534-1582) ended up committing seppuku ritual suicide following a rebellion by his vassal, Akechi Mitsuhide, who coincidentally died the same year.

Upon learning of Nobunaga’s demise during a visit to Sakai in today’s Osaka Prefecture, Ieyasu rushed to his base, Okazaki Castle, in present-day Aichi Prefecture, through the mountains of the nearby Koka and Iga regions.

“Ieyasu, as an ally of Nobunaga, desperately needed the cooperation of Koka samurai for his safe return to Okazaki,” Otomohara wrote.

“He was escorted not only by the Ohara family but also the Wada family (one of the Koka 21 families) and the Iga ninja clan. The total number came to 80.”

Hailing from Kyoto, Otomohara took it upon himself to uncover the scope of relations between the Koka clan and the founder of the Tokugawa Shogunate. He started working on the project in the 1990s while working in Aichi and Tokyo.

After relocating to Shiga Prefecture in 2000 due to his work, Otomohara began attending an annual gathering of the Ohara Domyoko family at its Otorijinja family shrine in the Kokacho Toriino district of Koka city.

The gathering, held each Aug. 3, provided Otomohara with an opportunity to listen to ninja tales from members of the ancient clan.

However, attendance at the gatherings is steadily falling off due to aging. Last year, only 25 people showed up, 15 fewer than 25 years ago.

That prompted Otomohara to begin working fulltime on his book so that “records will be handed down to posterity, as the practice of oral storytelling is beginning to die out.”

Though taking the form of a novel, Otomohara stressed that the work is “80 percent based on fact.”

It won the literary and art publication commendation at Shiga Prefecture’s literature festival in 2023.

Roughly 1,500 copies have been issued to date. The publisher, Sunrise Publishing Co. in Hikone, Shiga Prefecture, said orders are still coming in online.

“It has been relatively well received as an amateur novel, so I’d call it a kind of success,” said a Sunrise Publishing rep.

Otomohara’s next goal is a manga adaptation of his novel and an English version at some point. For now, he is working on summarizing the story and translating technical terms into English.

“I think I will be able to reveal my name by the time these works are published,” he said with a grin.

Otomohara is expected to deliver a speech at the Konan Tosho Koryukan library in the Konancho Fukawa district of Koka city in September.