When Surya the Sun God was defeated at the end of the Godswar, those Humans and Elves who stayed loyal to his rebellion were banished with him into the chains and hot imprisonment of his Realm. Surya's loyal elves never gave up their immortality and so stay forever imprisoned, although some do occasionally manifest as devils and demons in the Whorl when summoned by those desperate or cunning enough to try to pry open the bars of Surya’s Prison in the name of power.
Conversely, the humans among them remained mortal, and some managed to slip their prison entirely and live free among the other races of the Whorl. Their imprisonment in Surya's Realm did not come without cost, however. They were eternally changed by the chains and the bright heat into a new race known as the tieflings.
The original tieflings were merely a handful, rescued from damnation by powerful magic and clandestine acts performed by allies in the Whorl. In the millenia since, however, they have grown in number and in diversity. They are well aware of the stigma that their very existence represents, and they tend to respond to that stigma in varying ways. Some isolate themselves, either alone or in small communities of other tieflings. Others develop the social skills necessary to convince others that they aren't a threat, and that they have no loyalties to the Archfiend. Still others congregate in Ayodhya, the last place in the Whorl where devotion to Surya is not only tolerated but tacitly supported.