Tieflings

"The Chieftain Who Leads them," Thuel asked, "what do we know of him?"
“His name is Kathrik Mel. He inspires tremendous loyalty in the barbarians, an almost religious fervor.”
“He’s a demon?”
“I don’t think so. The Ghaash’kala call him a sak’vanarrak — it translates as something like ‘fiend-touched.’ A Karrn scholar coined the word tiefling. I think he’s some mixture of fiend and mortal, more like a savior than a god.”
Thuel frowned. “Their savior, our damnation.”
— James Wyatt, Dragon Forge

Tieflings rarely appear in the cities and towns of Khorvaire, with most dwelling at the fringes of civilization.
Many tieflings are born to the Carrion Tribes, the barbarians who live in the Demon Wastes. Such tieflings are touched by the fiendish forces bound beneath the Wastes, and the tribes consider them to be blessed. If you’re playing such a tiefling, why have you left the Wastes? It could be that you were destined for some evil purpose — perhaps even serving as an avatar for an imprisoned Overlord — and you are fleeing that destiny.
Other tieflings are shaped not by fiendish powers, but by the influence of the fiend-tainted planes. Such tieflings might be born in manifest zones, where a plane exerts influence over a region. Planar tieflings are oddities, often seen as strange but not necessarily evil.
Finally, the Venomous Demesne — a city-state hidden on the far side of Droaam — is populated by tieflings. These tieflings descend from Sarlonan mages who bargained with dark powers, with the Demesne’s lords being powerful warlocks and wizards. The Venomous Demesne has had no significant contact with the Five Nations and few know it exists.