Legend has it that during the Tran dynasty, after the naval battle of Bach Dang in 1288, fishermen discovered the body of a Tran general drifting to the island. The next day, the villagers went to the island to hold a burial ceremony for him.

Strangely, at the spot where the general lay, termites built up a mound that resembled a grave. Believing this to be a divine sign, the villagers erected a small temple to worship him. Later, they often saw the general appear in spiritual form as an old man with white hair and a long beard. From that time on, the fishermen respectfully called him "Cụ" (The Elder).

According to legend, during the Later Le dynasty, King Le once visited the Do Son region for an inspection. He spent the night on the island and dreamed of an old man with white hair and a long beard, who introduced himself as the "Island Deity."

The next day, the king recounted his dream to his entourage and declared, "If you are truly a deity, give me a sign." As soon as he finished speaking, a large fish leaped onto the boat with a mighty splash. Witnessing this divine response, the king conferred the title of "Great Deity of the Island" upon him and ordered the people to renovate the temple for worship.

During the Nguyen dynasty, while passing by Dau Island on an inspection trip, Emperor Tu Duc's boat was suddenly caught in strong winds and rough waves. Hearing the local officials report on the sacredness of the temple on the island, the emperor immediately went up to the temple to offer prayers.

Strangely, after the emperor finished his prayers, the sky suddenly cleared, the clouds dispersed, and the sea became calm. Emperor Tu Duc then bestowed upon him the title of "Nam Hai Than Vuong" (King of the Southern Sea).