Sai Thong Som
Sai Thong Som is traditionally identified as a mixed Mon–Tai monarch of Dvaravati's Devapura or Rajgir, which modern scholarship variously associates with Nakhon Pathom, Ratchaburi, or Khu Bua. His reign is attested in the Legend of the Arhat. According to this account, his father, Balidhiraja—a son of Kalavarnadisharaja—migrated from Sukhothai and forcibly deposed the incumbent monarch of Devapura around 687 CE. The throne was subsequently entrusted to his younger son Sai Thong Som.
Sai Thong Som is recorded as having had one sibling, his elder brother Balipatijaya, who acceded to the throne of Lavo in 700 CE, following the reign of their grandfather Kalavarnadisharaja. On the basis of onomastic analysis, the Thai historian Manit Vallibhotama has suggested that Sai Thong Som may have been born of a Tai queen consort. This interpretation gains plausibility in light of the historical context: after Kalavarnadisharaja’s enthronement as king of Lavo, he is reported to have extended his political influence northward into the Tai polity of Chiang Saen, which is mentioned as Nagendhara in the. It is within this framework of political expansion and consolidation that intermarriage between the Mon dynasty of Dvaravati and Tai elites likely occurred, cementing alliances through marital diplomacy and reshaping the ethnopolitical configuration of the region. By the mid-9th century, this dynamic had intensified; in 861 CE, records note that the Tai Yuan, or Tai Chiang Saen, had established a significant presence in Lavo and were increasing in political influence.
No definitive records survive concerning Sai Thong Som’s immediate successor. Nevertheless, archaeological and textual data suggest that by the early 8th century, corresponding with his reign, the political predominance of Dvaravati had entered a phase of decline.