“Of the many legends and stories told about Lê Lợi, the most famous concerns his magical sword.
A fisherman, who later joined Lê Lợi’s army, caught the blade of the sword in his net. The hilt was found by Lê Lợi himself in a banyan tree.
The stories claim Lê Lợi grew very tall when he used the sword and that it gave him the strenght of many men.
One day, not long after the Chinese had accepted Viet Nam as an independent country (thanks to Lê Lợi victories), Lê Lợi was out boating on the Green Water Lake. Suddenly a large turtle surfaced, took the sword from Lê Lợi’s belt, and dived back into the depths, carrying the glowing sword in his mouth.
Lê Lợi then acknowledged thet the sword had gone back to the Dragon King with the Golden Turtle, and he renamed the lake The Lake of the Returned Sword: Hoàn Kiếm Lake.”
Source: Friends of Vietnam Heritage, Hoan Kiem Lake – Its Legends and Temples