When Surya the Sun God was defeated at the end of the Godswar, those who stayed loyal to his rebellion were banished with him into the chains and hot imprisonment of his Realm. Certain loopholes in the language of Surya’s Prison, however, allow them to occasionally manifest as devils in the Whorl when summoned by those desperate enough for power to devy the gods. When such a summoned devil breeds with an elf or a human the offspring is a cambion, a creature tied intrinsically to Surya's Prison. When a cambion breeds with an elf or human, however, the offspring is a tiefling, a creature of infernal heritage who nevertheless possesses free will.
In the millenia since the first tieflings appeared in the whorl 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.
The majority of tieflings descend from one of the countless lesser devils populating Surya's Prison. Some, however, have the dubious honor of descending from one of Surya's great generals. These bloodlines are distinctive only insofar as an individual tiefling might care which devil gave birth to their lineage, although they do exhibit minor variations in physical characteristics and intrinsic magic.
You may select one of the "Tiefling Subraces" listed in Mordenkainen's Tome of Foes, p. 21. See Devils and Demons for details.