Mechanically-speaking, Tieflings in Pathfinder: Domain are identical to those found in the Pathfinder Advanced Race Guide p168. You can also find Racial Information on them on the d20pfSRD website.
XP Cost: 13,000
Tulun Tieflings: In a manner similar to Half Orcs and Eb’ari, Tieflings are half-breeds who struggle to fit into the myriad societies of Tulun. Finding it difficult to hide their Devlish parentage, most suffer a life of persecution or at the very least distrust and suspicion. Many fight against their natures and follow the path of righteousness, but just as many succumb to their darker impulses, much in the same way as Dhampir whom they share an affinity. Like them, they are a people of extremes: either valiant heroes or despicable villains, anything in-between feels half-hearted.
Place of Origin: Tieflings originally come from Geshar, also known as The Underworld; the lowest (both geographically and morally) part of Valael – The Godsrealm. Offspring of Devils and Humans, those who survived the harsh life of the Underworld were glad when they were pulled through into Tulun…initially.