“Of the many legends and stories told about Le Loi, the most famous concerns his magical sword. Similar to King Arthur and his sword Excalibur, Le Loi was said to have a sword of great power. The legend tells that the blade of the sword, inscribed with the words ‘The Will of Heaven’ (Thuan Thien), came from the Dragon King in his underwater palace. A fisherman, who later joined Le Loi’s army, caught the blade of the sword in his net. The hilt was found by Le Loi himself in a banyan tree. The stories claim Le Loi grew very tall when he used the sword and that it gave him the strength of many men.
The story relates how one day, not long after the Chinese had accepted Vietnam as an independent country, Le Loi was out boating on the Green Water Lake (Luc Thuy). Suddenly a large turtle surfaced, took the sword from Le Loi’s belt, and dived back into the depths, carrying the glowing sword in his mouth. Efforts were made to find both the sword and the turtle but without success. Le Loi then acknowledged that the sword had gone back to the Dragon King with the Golden Turtle (Kim Quy), and he renamed the lake ‘The Lake of the Returned Sword’ (Hoan Kiem Lake).” (source: en.hanoi.vietnamplus.vn)