Sertorius is set in the world of Gamandria, a place that draws on many historical cultures familiar to fantasy fans. Most notably the ancient Mediterranean cultures. But there are other influences as well and some of the strongest are the early Thai kingdoms of Sukhothai and Ayutthaya.
The earliest civilization in Gamandria, Nong Sai, draws from Thai history and myth. It is a fusion of the early kingdoms and the Thai version of the Ramayana, called Ramakien. This is the basis of Gamandrian history. It was a powerful cluster of Ogre kingdoms, occasionally united by a single ruler and always at war with Anumar, a rival power in the west. The two would clash in brutal wars fighting alongside their respective gods, Senga and Lorgo. Nong Sai was assisted by elven slaves, who were specifically created to serve the kingdoms. However they were released by the benevolence of Senga and this spread the culture further to the southwest. In the end, Nong Sai was destroyed when its final king stole Senga's power, killing the god and creating Sertori. This unleashed the wrath of the other gods who pulverized Nong Sai and cursed the Ogres.
Most of the present day cultures in Gamandria are surrounded by Nong Sai ruins. Many build their cities on top of them, incorporating the ancient architecture into their own designs. The language of Nong Sai, Singh, remains influential and most people speak it in bits and pieces. The elves carry on and in some cases deliberately resurrect elements of Nong Sai culture. The Ogres who are cursed to wander, also continue the traditions of their great civilization.
I was inspired by many different sources. For the myth, I mainly took inspiration from temple art and from oral retellings of Ramakien when I worked at a Thai restaurant. But I also used translations of epic poems that inspired me. One that I quite enjoyed is the Isan folk tale Phadaeng Nang Ai translated by Wajuppa Tossa. For history I found both Chris Baker's A History of Thailand and David K. Wyatt's Thailand: A Short History to be useful. Another helpful resource was a book called South-East Easia: Languages and Literatures, a select guide (Herbert and Milner). Thai movies, music and television shows also provided some inspiration.
If you look at the images of Ogres in the Sertorius rule book the Thai influence is pretty apparent. In the above illustration of the Ogre King killing Senga, the backdrop is based on Sukhothai temples. The clothing is based on Thai design and the magic symbols floating around Senga were inspired by Thai gold leaf images. Present-day Ogre chieftains in Gamandria wear crowns based on the Thai chada (see the illustration below).
And it isn't only the Ogres or elves who continue Nong Sai Traditions. The humans living in elven influenced regions have adopted the ways of Nong Sai. This is most notable in Khata and the surrounding areas. Below the image of the Ogre chieftain is a human being crowned queen in a city within the sphere of Khata's influence.