Mechanically-speaking, Merfolk in Pathfinder: Domain are identical to those found in the Pathfinder Advanced Race Guide p194. You can also find Racial Information on them on the d20pfSRD website.
XP Cost: 9,000
Tulun Merfolk: Once the predominant race in the Emerald Ocean, Merfolk had free reign of hundreds of miles of open ocean before the time of The Reckoning and the sudden arrival of the Kingdom of Kajir. Initially retreating from the turbulence and weather disturbances on the surface, the vast majority of Merfolk initially kept away from the island, but it did not take long before curiosity got the better of them and they investigated. Over 80 years later, the Merfolk are considered an important part of the water-based political faction of the kingdom of Kajir, with many of the more adventurous of their people leaving their lives beneath the ocean to represent the ocean-dwellers in specially constructed habitats.
Place of Origin: Merfolk originally evolved beneath the ocean in the time known as The Mythic Age, and have remained there almost entirely ever since. Many (especially Adventurers) are now found more commonly in the Kingdom of Kajir and its outlying territories, acting as guardians of the waves, pirates or glory-hounds.