After defeating the last king of the Hùng era, putting an end to a dynasty that reigned the kingdom for so long, Thục Phán An Dương Vương unified the two countries of Âu Việt and Lạc Việt, establishing it into a new kingdom and named it Âu Lạc. He then proclaimed himself King An Dương Vương and Âu Lạc became the country’s new center of power.

With the strategic and geographic importance of Cổ Loa, he declared it as the kingdom’s capital and ordered for a fortress to be built around the city for its walls to keep the capital safe from invaders. However, the construction of the spiral-shaped citadel became difficult to complete and each time they succeed in putting up a wall, they crumble. According to a prophecy, a coalition of evil spirits of the Hùng Vương descendants seeking revenge for the loss of their kingdom prevents the walls from ever being put up.

Seeking for the completion of the fortress to help protect his reign, King An Dương Vương burnt incense, prayed, made offerings, and evoked all the gods to grant him his desire. In answer to his prayers, the Gods sent a magical golden turtle to aid the King in accomplishing his wishes. After the walls of the spiral fortress, Cổ Loa Thành, solidly stood, the turtle was summoned back by the Gods. As a token to the king, he gave one of his claws and instructed An Dương Vương to use it as a trigger of his royal crossbow. Such, he said, will keep his kingdom invincible.

And indeed, with the help of his magic weapon, King An Dương Vương succeeded in protecting his kingdom for a long time. Many times, the ambitious Chinese warlord Triệu Đà attempted to invade Âu Lạc, but he never succeeded. To devise his scheme, he negotiated a peace treaty with Âu Lạc. And shortly after, he sent his son, Trọng Thủy, to the King's court to ask for the hand of Princess Mỵ Châu for marriage. The King approved and Trọng Thủy came to live in the kingdom.

Through Mỵ Châu, Trọng Thủy learned the secrets of the King and how the magic crossbow kept their kingdom protected. Secretly, he replaced the golden turtle’s claw with an ordinary claw and seeked permission from his wife to see his father. Before he left, he made a promise to Mỵ Châu that in case a conflict arise, he will come back to look for her. Mỵ Châu, in return, promised her husband that she will drop goose feathers from her blanket along her track so Trọng Thủy will be able to find her. Trọng Thủy bid his farewells and took the magic claw with him back to his father.

Having possessed the turtle’s magical claw, Triệu Đà then raised a sudden attack against Âu Lạc. But King An Dương Vương was confident that his crossbow would once again save his kingdom. He calmly faced Triệu Đà's army as they arrived at the gate of his fortress, took out his magic crossbow and fired at the invaders. But nothing happened! The king then realized that his golden claw that used to kill thousands with a single shot had been replaced. Unprepared, the king and his army fled in panic.

With Princess Mỵ Châu behind his horse, the king fled to the south. But when he reached the seashore, there was no ship in sight. In desperation, the king again called on to the Gods for help. The golden turtle, sent by the gods, emerged from the waters and appeared before An Dương Vương. The turtle told the king of his daughter’s betrayal and instructed him to destroy her before he would be saved. So the King pulled out his sword and beheaded Mỵ Châu. With that, the golden turtle took the King below the waters and disappeared.

Guided by the goose feathers, Trọng Thủy followed the trail and arrived the seashore shortly, only to find the body of his beloved wife lying in a pool of blood and King An Dương Vương nowhere to be found. Mỵ Châu’s blood flowed down the ocean floor where the oysters swallowed it, amazingly transforming them into pearls. In deep remorse and grief over the death of Mỵ Châu, he drowned himself as well in order to be with her in eternity.