Vat Visounnarath (or Vat Visoun for short) was built between 1500/1520 during the reign of King Visounnarath. It is an important temple in Lao history because the Prabang, the fine gold Buddha image, the religious symbol of the Kingdom, was enshrined here from 1504 to 1715 A.D. and from 1866 to 1874 A.D. In 1942, it was turned into a Museum of Religious Arts, and housed collections of Buddha images and religious artifacts from the fourteenth century.
Within its precincts stands the gigantic Lotus Stupa called That Pathum but is more commonly known as "That Markmore" because its shape resembles a watermelon (Markmore is a Lao word for watermelon). The stupa was originally constructed around in 1503 AD. Its interior was originally filled with hundreds of small Buddha images fashioned in precious metals and crystal. When the Chinese Haw sacked the city's temples near the end of the nineteenth century, they smashed open the stupa and stole most of the images. What remained is now on display in the Royal Palace Museum.
Inside the large sim is a typical alter with the typical large seated Buddha image surrounded by many standing images. In addition to these, around the outer wall of the sim behind the alter are arrayed many more standing Buddha images. Vat Visoun is adjacent to Vat Aham, so a visit to one typically takes in the other. You can say "Vat" or "Wat", they both mean temple but "Wat" is a Thai word and "Vat" is a Lao word.