Hong Kong (CNN) When feudal Japan's most powerful warlord Nobunaga Oda met Yasuke, a black slave-turned-retainer, in 1581, he believed the man was a god.

Oda had never seen an African before. And like the locals in Japan's then-capital of Kyoto, he was awed by Yasuke's height, build and skin tone, according to Thomas Lockley, the author of "African Samurai: The True Story of Yasuke, a Legendary Black Warrior in Feudal Japan."

"When Yasuke got to Kyoto (with Jesuit missionaries), there was a massive riot. People wanted to see him and be in his presence," says Lockley, who spent nine years researching and writing the book, which was published last month.

Oda believed Yasuke to be either a guardian demon or "Daikokuten," a god of prosperity usually represented by black statues in temples. He tried to rub the pigment from Yasuke's skin, believing it was black ink. Once convinced Yasuke was real, he immediately threw a feast in his honor, says Lockley.

In an era racked by political espionage, merciless assassinations and ninja attacks, Yasuke was seen as an asset. Nobunaga soon made him a samurai -- even providing him with his own servant, house and stipend, according to Jesuit records.

Today, Yasuke's legacy as the world's first African samurai is well known in Japan, spawning everything from prize-winning children's books to a manga series titled "Afro Samurai."

And his legacy continues to spread worldwide.

Earlier this month , "Black Panther" star Chadwick Boseman announced he would play Yasuke in a Hollywood movie scripted by "Narcos" co-creator Doug Miro.

Lockley says his story has reemerged just as homogenous Japan reexamines the concept of multiculturalism in the run-up to the 2020 Tokyo Olympics.

Early years

Yasuke's origins remain a mystery as historical sources are scant. While some researchers believe he was from Mozambique , others suggest Sudan.

Lockley suspects that Yasuke was abducted from his family as a child by Arab or Indian slave traders and trafficked through Arab countries and across the Indian Ocean. He likely worked as a slave and trained as a child soldier who fought in Gujarat and Goa in India, before being hired as a valet by Jesuit missionaries from Portugal.

A Portuguese black ship arrives in Japan from Goa and Macau.

It's where Lockley speculates that Yasuke met Alessandro Valignano, the most powerful Jesuit missionary of the day in Asia, who made him his valet and bodyguard.

The pair and their entourage arrived by ship in 1579 at the port of Kuchinotsu in Nagasaki, on the southern Japanese island of Kyushu, according to Lockley.

Valignano, who had spent six years traveling from Rome via countries such as Portugal, Mozambique, India, Malaya and Macau, hoped to convert thousands of Japanese to Christianity.

But his mission would not be easy.

Ninjas, warrior monks and samurai

When he arrived in Japan, the country was embroiled in a brutal civil war that ended only in 1603.

The period -- known as the " era of warring states " -- saw hundreds of strongmen from mini-states across the country battling for power.

A semblance of peace was restored when the remaining local feudal warlords, or "daimyo," sought to unify Japan.

Nobunaga Oda became the most powerful among them. He controlled Kyoto, the dominant center of the country, and is viewed as one of Japan's three unifiers along with Ieyasu Tokugawa and Hideyoshi Toyotomi.

Nobunaga Oda was considered the most powerful warlord in Japan.

But even his ascent did not stop minor warlords and bands of radical armed monks and bandits vying for territory, according to Lockley. Valignano needed protection.

Yasuke was tall and used his military experience to detect risks for the Jesuits as they formed alliances with local warlords, says Lockley. He trained other militiamen and likely learned new techniques himself, including Japanese martial arts and sword skills.

Such skills would later appeal to Oda, who also looked to Yasuke -- by then conversant in Japanese -- for news about the wider world.

"Yasuke was initially viewed as a source of entertainment as he was a novelty, but within a month he'd become a valued samurai and member of Oda's entourage," says Lockley. "According to the sources, Oda just loved talking with Yasuke."

At the time, the samurai -- groups of warriors well versed in warfare and the arts -- formed the ruling class in Japan.

Given that there are no records of how much Yasuke earned, Lockley says it's hard to know how highly he ranked. He speculates that the African was the equivalent of a page or bodyguard to Oda.

But while Yasuke became Japan's most famous foreign-born warrior, his time with Oda was short-lived.

From samurai to ronin

In 1581, Yasuke joined Oda's forces in their invasion of Iga province, according to Lockley.

Oda attacked the mountain-ringed province, a ninja hotbed with 40,000 to 60,000 troops, and conquered it following a failed attempt by his son Nobukatsu in 1579.

It was, says Lockley, Yasuke's first military campaign under Oda.

His second and last such campaign was in June 1582 when Oda's samurai general, Mitsuhide Akechi, attacked Oda's residence in Kyoto.

Nobunaga Oda was forced to commit "harakiri", a form of Japanese ritual suicide by disembowelment after his defeat in the Battle of Honno-ji.

The attack, which triggered what was known as the Battle of Honno-ji Temple, put an end to Oda's plans to consolidate power in Japan.

Facing defeat, Oda ended his own life to avoid losing his honor. He performed a ritual called "sepukku" which saw him stab a short sword into his stomach, slicing horizontally while his attendant Ranmaru Mori lopped off his head.

Legend has it, says Lockley, that Oda's last order to Yasuke was to take his sword and his decapitated head to his son.

"Oda's head couldn't fall into someone else's hands. Yasuke's job was to keep the clan power," says the author.

After Oda's death, records on Yasuke became scarcer. The last possible references to him, according to Lockley, were from Jesuit accounts in 1582.

According to Gary Leupp, a professor of history at Tufts University, Yasuke was taken prisoner by Oda's enemies but later released because he was not Japanese. Yasuke had become a "ronin" -- a samurai without a master.

Lockley speculates that Yasuke could have either resumed his previous role of guard to Jesuit missionaries or become a sailor or pirate.

Legacy

While Yasuke's existence has gone down in the history books, he was by no means the only foreigner in Japan.

At the time, Kyushu was home to a large population of Koreans and Chinese. Many Europeans, Indians and Africans also passed through the country.

Full Japanese folding screen showing the arrival of Portuguese to Nagasaki.

Their presence is documented on the handcrafted folding screens of the era, which depict their arrival on large black ships and their life alongside the locals.

Such ornately decorated screens belonged to the upper classes and were produced in the early 1590s. One portrays a wrestling match between a black man and a Japanese warrior, which Lockley assumes are Yasuke and Oda.

"Yasuke really comes to the fore because he served Oda. We have sources on his life, name, deeds and character," says Lockley.

"Others like him weren't that well documented, we can't bring a picture of their lives."

Yasuke's life has often been reimagined through fiction.

In 1968, author Yoshio Kurusu made it the basis of a prize-winning children's book called "Kurosuke." In more recent years, there have been Japanese TV historical dramas and comic books.

And as debate on multiculturalism and diversity intensifies in the nation, Lockley says it's the right time for Yasuke's story to be told again.

"There's still a kind of romance and mystery to the story of someone who escaped slavery and was raised to foreign heights next to the prime ruler of Japan," says Lockley.

"It feels like the age where he'll get the attention he deserves."