Ngoc Son Temple was built in the 18th century on Jade Island in the centre of the ‘Lake of the Returned Sword’ or Hoan Kiem Lake. Legend describes how an emperor was once given a magical sword which helped him defeat the Chinese Ming Dynasty and in doing so saw the return of the Golden Turtle God to the lake.
Today ‘Turtle Tower’ stands close to the lake in memory of this legend. There are also endangered large soft-shell turtles swimming in the lake, and to see one of these gentle giants is considered very auspicious. The name of the temple translates to ‘Temple of the Jade Mountain’ and is predominately dedicated to war hero General Tran Hung Dao who defeated an armed force of 300,000 soldiers sent by Mongolian Emperor Kublai Khan in the 13th century to invade Vietnam.
Also inside the pagoda are a large bronze bust and other deities. There are altars dedicated to Tran Hung Dao, some ancient artefacts including ceramics and a preserved specimen of a giant turtle found in the lake weighing 250kg.
http://www.vietnam-guide.com/hanoi/hoan-kiem-lake.htm