Śarīra (佛舍利) are pearl-like, indestructible beads of a variety of colours found among the ashes of cremated Buddhist masters. Long venerated as sacred relics by Buddhist monks, the Japanese scholar Abe no Nakamaro discovered that they were in reality remains of Isu and receptacles of their memories.
Among the first śarīra to be recovered by humans were those found among the ashes of Shakyamuni, the founder of Buddhism, upon his cremation. His disciples collected them and sanctified them. Few knew of Shakyamuni's true nature as a "Precursor", at least according to Abe no Nakamaro.
Over the centuries, more and more śarīra were accumulated as Buddhism rose in prominence in India. As the religion spread along the Silk Road into Central Asia, so followed scores of śarīra. They poured into China via the Western Regions during the Han dynasty, entering into the hands of the earliest Chinese Buddhist priests. These master monks resolutely protected the śarīra in their temples from the chaos and non-stop warfare that followed the collapse of the Han, passing this duty from generation to generation among their communities well into the Tang dynasty.
In the 8th century, one of the monks who inherited śarīra was Jianzhen of Yanguang Temple in Yangzhou. He enshrined them at this temple but over the years grew more and more apprehensive about developments at court. By 754, he had learned of the existence of the Golden Turtles, a secret organization which had infiltrated the government to manipulate it for their own ends. Predicting that a crisis would soon befall the Tang and recognizing the potential to abuse the śarīra, he decided to spirit them away to Japan to prevent them from falling into the wrong hands. After five failed attempts to cross the ocean, he entrusted this mission to a dear friend, the Japanese scholar Abe no Nakamaro who had studied, lived, and worked in China for decades but longed to return to his childhood home.
In 753, the two embarked on a voyage back to Japan with the Japanese diplomat Fujiwara no Kiyokawa, using the official status of Fujiwara's mission as cover for the śarīra they were transporting in secret. As had been the case previously, their four ships were beset by a storm along the way. Fujiwara and Nakamaro's ship ran aground, and Nakamaro awoke on the shore of Annam, unaware of the fate of Jianzhen and the śarīra. In Annam, his crew was attacked by agents of the Golden Turtles who had learned of their secret mission. Most of the crew were slain, and the Golden Turtles relentlessly pursued Fujiwara and Nakamaro throughout Annam, not realizing that the śarīra had never been with their ship.