Ryuoson Zenpoji is one of the Three Main Soto Zen Buddhist temples in Japan, along with Myogonji Temple (Aichi Prefecture) and Saijôji Temple (Kanagawa Prefecture).
At the time it was first built in the 10th century, it was named Ryûgeji 龍華寺: "the temple of the Lotus and the Dragon." Even if it wasn't a Zen temple initially, its original name shows the dragon-god worshipping practice was already existing since the origin of this temple. It received its current name of Zenpoji after its rehabilitation into Zen Buddhism in the 15th century.
Because it is one of the most ancient dragon-worshipping temples in the North, many fishermen and sailors from the Tohoku region come to Zenpoji to pray to the dragon god. Dragons are water gods in Japanese culture, and they are thought to interfere with fishing, healing, travel, agriculture, and fertility.
Zenpoji played an essential role in the development of the Kitamaebune maritime road during the 18-19th century. Merchants and fishermen who traveled by sea sought the dragon god's protection during their journey. With all the donations made by those wealthy visitors, the temple could build in 855 the Gohyakurakan-dô, where the 531 statues called "Gohyaku Rakan" (the 500 Arthats) are exposed for people to mourn their lost ones. The Gohyaku Rakan all have different expressions and different faces so that people who, at the time, didn't have pictures to recollect about their loved ones could pray in front of a statue looking like the deceased person.