After helping An DuongVuong - king of Au Lac nation - build Co Loa citadel, saint Kim Qui* offered him one of his claws to make a trigger of crossbow to protect the citadel from enemies. As the saint's words this crossbow was magic one. Every arrow shot from the crossbow with magic trigger would hit a thousand of enemies at the same time.
The king chose Cao Lo, one of the mandarin's household butlers, who was the most skillful crossbow maker in the country to be in charge of the heavy responsibility. However, this kind of weapon only suited to athletes to use. The king extremely treasured the crossbow so he hung it in his sleeping room.
At that time, Trieu Da was the governor of a country adjoining Au Lac at the north. He had failed to occupy his neighboring nation for many times so he tried to guard his country by all means and waited for the right time. He then sent his son named Trong Thuy to Au Lac to seek a marriage alliance.
Trong Thuy then met My Chau, a dear daughter of An Duong Vuong. She was the most graceful lady of the country at that time. They were soon in love with each other and to be side by side to every where in the citadel. Witness the passionate love of the young couple, the king doubtlessly allowed Trong Thuy to take his dear daughter as a wife.
One night, when sitting in the garden in the moonlight, Trong Thuy asked his wife why there was no one who could defeat the country and if there was a secret. Honestly the innocent princess replied her husband that there was nothing but solid defence works in the citadel and a crossbow with a magic trigger which was kept in the king sleeping room. Trong Thuy was so surprise as if it had been the first time he heard that. The princess immediately took the crossbow out and showed it to the man. She also told him the way to use the crossbow.
One day later, Trong Thuy asked the king for permission to visit his father. He retold his father what he had known and they all agreed to find someone to make trigger reproduction. Finally Trong Thuy came back; he was offered a feast to celebrate the occasion of reunite. Trong Thuy drunk half-heartedly while An Duong Vuong and the princess so enjoyed the feast that they both were drunk at the end. Catching the chance, Trong Thuy secretly broke into the king's room and exchanged the magic trigger by a false one.
Once again Trong Thuy asked the king for permission for returning to his country for some days. The two then were loath to path with each other. Trong Thuy said to his beloved wife that he had to come back to depart a trip to the remove place in the North and it was hard to know when they could met again because of the troubled times. The poor wife released her husband that she had a fur coat so she would make marks on the way she went through with fur in order that he could find her. She then sobbed her heart out.
In a few days time Trieu Da rose troops to Au Lac. When hearing the news, An Duong Vuong didn't take any precaution against. He waited until the enemy reached to the citadel and asked his butler to bring the crossbow to fight back. Unfortunately it wasn't magic one. The citadel at last was occupied; An Duong Vuong had to evade with his dear daughter on a horse's back. The princess remembered what she had told to her husband before they separated so she took the fur coat along with her and marked the way with fur.
King An Duong Vuong and his daughter were on the horse's back for days, they had went through many rocky mountains and many bumpy paths and reach to the seashore while the enemy was tracing behind them. The king got down, turned his face to the sea and prayed saint Kim Qui with supplication. A whirlwind rose to replied the king's words. After that the saint appeared and told him that the enemy was at his back. An Duong Vuong woke up to reality. He drew sword out and cut off his dear daughter's head then jumped into the sea.
Trong Thuy at that time followed the marks to the seashore and found his wife lying dead on the grass with her unchangeable appearance. He burst out crying then buried her in the citadel and jumped into the well where his wife usually washed her hair.
Nowadays, in Co Loa village, there were a temple of King An Duong Vuong and a well called Trong Thuy's in front of the temple. It is said that when My Chau died, her blood leaked into the sea, oyster ate it then born precious pearl. If this kind of pearl was washed by water from Trong Thuy's well, it would be much brighter.

* In Vietnamese folk literature Kim Qui was a saint with an appearance of a tortoise.