Legacy of the Last War
Most of Eberron’s tieflings arose from the corrupted bloodlines of the Sarlonan nation of Ohr Kaluun. A culture steeped in arcane lore and obsessed with eldritch knowledge, many of its leading citizens entered into pacts with devils. What they sought—knowledge, power, and immortality—was secondary to any consequences their descendants might face. When the Inspired sought to wipe out unauthorized magic from Sarlona, Ohr Kaluun fell, but not before many tieflings fled to other lands.
Many tieflings who fled Ohr Kaluun went to Khorvaire. The largest population of tieflings founded the Venomous Demesne in the Demon Wastes, but tiefling populations also survive in the Shadow Marches, Droaam, the Eldeen Reaches, and even rural parts of Aundair.