From 1297 to 1301, Lao troops under Mongol command invaded Dai Viet but were repulsed by the Vietnamese. Troops from Muang Sua (later Luang Prabang) conquered Muang Phuan in 1292-97. In 1308 Panya Khamphong seized the ruler of Muang Phuan, and by 1312 this principality was a vassal state of Muang Sua.

Mongol overlordship was unpopular in Muang Sua. Internal conflicts among members of the new dynasty over Mongol intervention in their affairs resulted in continuing family upheavals.

Panya Khamphong exiled his son Fa Phi Fa and most likely intended to leave the throne to his younger grandson, Fa Ngieo. Fa Ngieo, involved in various coups and coup attempts, in 1330 sent his two sons to a Buddhist monastery outside the Mongol realm for safety. His youngest son Chao FaNum, was said to have been born with teeth.

The brothers were kidnapped in 1335 and taken to Angkor, where they were entrusted to King Jayavarman Paramesvara, whose kingdom had acknowledged the Mongol since 1285.

The younger brother, Fa Ngum, married one of the king's daughters, Nang Yordkeo (Yotkeo) and in 1349 set out from Angkor at the head of a 10,000-man army and 500 elephants

His conquest of the territories to the north of Angkor over the next six years reopened Mongol communications with that place, which had been cut off. Fa Ngum organized the conquered principalities into provinces, and reclaimed Muang Sua from his father and elder brother.
He was frequently at war with both the kingdom of Ayutthaya and Annam.
Fa Ngum was crowned king of Lan Xang at Vientiane, the site of one of his victories, in June 1354. Lan Xang extended from the border of China to Sambor below the Mekong rapids at Khong Island and from the Vietnamese border to the western escarpment of the Khorat Plateau.

Largely by the efforts of his Khmer queen,who persuaded him to send to Angkor (1357) for Theravada Buddhist priests, scriptures, and a statue of Buddha called the Prabang, Theravada Buddhism was adopted during his reign.

After his queen died, Fa Ngum neglected his government and allowed Kwan Meesa to alienate people with his tax collection and favoritism. Eventually nobles forced Fa Ngum into exile in 1371.

He was the first Lao King of LanXang and Muang Sau was renamed Luang Phrabang. In 1353, the territories of which modern Laos is now part of, was named LanXang Hom Khao ( Million elepants - white parasol).

He died in 1573, ruled for 20 years and was 58 years old.

His son Prince Ounkham, born in 1536, took the throne and was named as King Samsenthai.