After King An Dương Vương built Cổ Loa citadel, the god of Kim Quy (Gold Turtle) gave the king a toenail of himself to make a lẫy (an important part of the crossbow) of a crossbow to protect the citadel. The god said if the crossbow having the lẫy made of his nail it would shoot a hundred times, hit the target enough hundred times and one time would kill thousands of enemies. An Dương Vương chose a very good man at making crossbows named Cao Lỗ and asked him to make the crossbow. He did hard for many days to finish. The crossbow was very large and very heavy, different from the regular crossbows, which had to be lifted by an athlete. An Dương Vương was fond of the magical crossbow very much, always hanging next to the place he lay.
At that time, Triệu Đà (Chinese: Zhao Tuo) was the lord of Nam Hãi land (now belong to China), several times brought troops to rob Âu Lạc land (now belong to Vietnam), but because An Dương Vương had the magical crossbow, the Nam Hãi army was killed a lot, so Đà had to try to wait for the opportunity. Triệu Đà found that using the army did not benefit, so he asked to make peace with An Dương Vuong and sent his son Trọng Thủy to have friendship, but really had conspiracy breaking the magical crossbow.
During the days of traveling to have friendship, Trọng Thủy met Mỵ Châu, the beloved daughter of An Dương Vương, a young lady having a wonderful beauty. Trọng Thủy fell in love with Mỵ Châu at first sight, Mỵ Châu fell in love with him, too. The two became closer and closer, there was nowhere in Cổ Loa where Mỵ Châu did not show her lover to see. An Dương Vương did not suspect anything. Seeing the young couple loving each other, the king immediately gave his daughter to Trọng Thủy as a wife.
A moonlit night, Mỵ Châu and Trọng Thủy sat on a white stone in the middle of the garden, looking together at the highest wall of the citadel. In the friendly story, Trọng Thủy asked his wife, “My dear, is there any secret in Âu Lạc that no one can beat?” Mỵ Châu answered, “Âu Lạc had high walls, deep moats, and a crossbow that shot thousands of enemy soldiers a time, so who could beat?”. Trọng Thủy surprised, pretending to have heard the magical crossbow for the first time. He offered to see the crossbow. Mỵ Châu did not hesitate, ran right into the place where her father usually lay, took the magical crossbow to show her husband. She showed him the lẫy of the crossbow which was made of the god Kim Quy’s nail and told Trọng Thủy how to shoot. Trọng Thủy attentively listened, staring at the lẫy, looking at the crossbow frame for a long time, then gave the crossbow to his wife to put in its place.
The next day, Trọng Thủy asked An Dương Vương’s permission to come home to visit his father. Just having arrived at the place, he immediately told Triệu Đà about the magical crossbow. Đà ordered a workman specializing in making crossbows to make a lẫy the same as the one of An Dương Vương. After the man faked finished, Trọng Thủy hid the crossbow inside his shirt and went back to Âu Lạc.
An Dương Vương, who always followed his daughter’s idea, found that when she met her husband, she was happy and happy. He immediately orders his servants to organize a party for them having fun. Trọng Thủy drank in moderation, while An Dương Vương and Mỵ Châu drank so much. when his father-in-law and wife was drunk, Trọng Thủy sneaked into the room to remove the lẫy made of Kim Quy’s toenail and replaced the false lẫy made of the common turtle’s nail.
The next day, seeing her husband seem restless, Mỵ Châu asked her husband, “Is there anything you worry about? Trọng Thủy replied, “I am about to go far, Father told me to go right back in order to go to the North, the North is very far away.”. Mỵ Châu was both sad and quiet, Trọng Thủy continued, “Now we are about to be apart, not knowing when we will meet again! If a war disaster happens, I do not know where I will look for you? Mỵ Châu said, “I have a goose feather coat, I will sprinkle the feathers along the way, let run after the trace.” After saying, Mỵ Châu sobbed.
Returning to Nam Hải, Trọng Thủy handed the lẫy of Kim Quy to his father, Triệu Đà was very happy and exclaimed, “This time, the land of Âu Lạc will belong to my hand”. Only a few days later, Triệu Đà ordered an army to attack Âu Lạc.
Hearing the news, An Dương Vương still trusted the magical crossbow, no defense at all. When the time the enemy troop came so close, An Duong Vuong ordered to take the magical crossbow to shoot and did not see the effection. Triệu Đà army broke the gates and rushed in. An Dương Vương hurriedly boarded his horse, brought Mỵ Châu, and exited through the back gate. Sitting behind his father, Mỵ Châu plucked the feathers in her coat sprinkled along the road.
The road was rugged, the horse ran for days and nights. Arriving Dạ Sơn mountain next to the coast, the father and daughter were about to get off the horse to rest when the enemy army was near. Seeing the winding mountain road sloping upside down, the afternoon shade had descended, there was no way to run, An Dương Vương turned the face out to the sea, praying for Kim Quy to protect him. The king had just made a prayer when a wind of dust rose up, shaking the mountains and forests. Kim Quy god appeared, telling An Dương Vương, “The enemy is behind the king”. When An Dương Vương came to his senses, he drew the sword to cut Mỵ Châu, then jumped down the river to kill himself.
Triệu Đà’s army pulled in to occupy Cổ Loa citadel, and Trọng Thủy alone a horse followed the goose feathers to look for Mỵ Châu. Approaching the coast, he saw his wife’s body lying on the grass, but her beauty did not fade. Trong Thuy cried loudly, collected the body and brought it back to bury in the citadel, and then plunged his head into the well in the citadel where My Chau had used to bathe.
Today, in Cổ Loa ancient citadel in Cổ Loa village, Cổ Loa commune, Đông Anh district, Hanoi city, front of An Dương Vương temple, there is a well called Trọng Thủy Well. Legend said that when Mỵ Châu was killed by her father, her blood flowed into the sea, oysters ate it so they had pearls and someone took pearls to wash them with the well water in Cổ Loa citadel, they would be extremely bright.