(The following is from Spirit Worlds, my forthcoming book about Cambodian religion and belief. Part of that system of beliefs is the national myths that give meaning to being Khmer).
In India there lived a prince, Preah Thong by name, who was told in a dream to take a ship and sail to a golden land in the east where he would establish a great kingdom. So the prince prepared his ship and crew and set forth, and after many days he came upon a beautiful and uninhabited island. Preah Thong named it Nokor Kok Tlok, Kingdom of the Tlok Tree, after a large tlok tree by the shore. Possibly feeling he had done enough for one day Preah Thong fell asleep under the tree, and when he awoke it was night. The moon was full and he gazed on the sea, wondering about his new kingdom, and especially about the lack of people, and as he watched he saw human figures emerge from beneath the waves.
The beings spread a feast under the trees, and the men fell to sport, wrestling and sword-playing on the sand, while the women strummed musical instruments and sang sweet songs. All were comely, but in their midst was a lady more arresting than any. Overcome by her beauty, Preah Thong stepped out from his hiding place and introduced himself, asking who she was, and how it came to be that she and her people lived beneath the waves.
The lady graciously permitted him to know that she was Neang Neak (Lady Naga), the daughter of the naga king, that this was the land of the nagas, and that she and her court visited the beach every full moon night for their pleasure. She informed him also that it was a serious breach of protocol to speak to royalty before being spoken to, and that he had broken this rule, but as she could see he was a stranger and ignorant of naga etiquette she would permit him to inform her of his name and family before ordering his execution.
Quickly Preah Thong told the naga princess of his foreign origins and princely station and declared his undying love, and after further gentle words the proud princess melted and agreed to take him to the underwater kingdom of the nagas so that he could ask her father for her hand in marriage.
And so Neang Neak took Preah Thong to the kingdom of the nagas, the prince holding her scarf. For three days they celebrated their wedding, and at the end of that time the naga king swallowed the water that had covered the land and Preah Thong and Neang Neak ruled together over the Khmers, who were descended from the arriage of the noble Indians who had accompanied Preah Thong and the beautiful naga-folk who waited upon the princess.
In another version of the legend the Indian prince is a Brahmin named Kaundinya and the naga princess is named Soma. The plot is very similar to the story of Preah Thong and Neang Neak except that the island is apparently upstream in the Mekong, and Kaundinya defeats Soma in battle before she agrees to marry him. In yet another version the prince was called Kambu, from which comes the name Kambuja, Cambodia.
The great enemy of the Khmers were the Cham, whose kingdom of Champa lay along the central coast of present-day Vietnam. In the year 657 a Cham king named Prakasadhamma left an inscription in his capital telling how the Brahmin Kaundinya ‘planted his spear’ (settled) in Kambuja and took Soma, daughter of the naga king, as his wife. Prakasadhamma mentioned this because, although a Cham king and a Cham by blood through his mother, he was a descendant of Kaundinya and Soma through his father, who was a Khmer prince. The inscription is important because it is the earliest mention in real history, as opposed to myth, of the name Kambuja.
The marriage of human prince and naga princess sounds fanciful, but the Chinese traveller Zhou Daguan says this in the record of his stay at Angkor shortly before the year 1300:
Inside the palace there is a gold tower, at the summit of which the king sleeps at night. The local people all say that in the tower lives a nine-headed snake spirit which is the lord of the earth for the entire country. Every night it appears in the form of a woman, and the king first shares his bed with her and has sex with her. […] If for a single night this spirit does not appear, the time has come for this […] king to die.
The tower is probably Phimeanakas, the only structure still remaining inside the palace compound at Angkor. I climbed to the top one cloudy and tourist-free day in the monsoon season, but found no naga-maiden awaiting me. Given what happened to the mythical Leper King, husband to the naga-princess and son-in-law to the naga-king, this may be just as well. That, however, is another myth.