The Sarlonan nation of Ohr Kaluun was well known amongst it's neighbours for practicing dark magic. The mage lords of Ohr Kaluun forged pacts with fiendish spirits and utilized the darker powers of the planes to achieve their goals, and over time this twisted them physically into being we might now recognise as tieflings.
During the Sundering, when the Unity of Riedra spread across the continent, Ohr Kaluun was utterly annihilated. A small community fled into Khyber, however, and emerged somewhere in western Khorvaire. Here, they created a secret enclave and worked tirelessly to regain their lost power. These Kaluunites used their powerful magic to hide their settlement, and inspired fear in those who lived in proximity to them, earning the name "The Venemous Demesne" from their neighbours.
The Kaluunite tiefling's were content to remain isolated until they received a visit from Sora Teraza, who was able to circumvent their many wards. She spoke to the Council of Four, and though nobody knows what exactly was said, they declared their allegiance to the Daughters of Sora Kell just days after she left.
Kaluunite Tieflings are the closest to the tieflings of other D&D settings, shaped by pacts and interactions with fiends.