The Arudien Empire
The Arudien Empire came to power through their demonic patron Arud, the Master of Chains, the Lord of Suffering, the Diseased. The conquest of the continent by Arud's followers was quick an brutal.
Life in the empire after the conquest remained brutal. There was a clear distinction between the conquerors and those subjugated, as Arud's loyal followers received a demonic blessing, and thus Tieflings were born. Slavery was legal and widely used to forcefully resettle people across the entire continent.
The empire only came to ruin when the gods decided to intervene. In an event, called the Convergence the six major deities Mael, Taith, Worem, Tiem, Fenya and Sev destroyed Arud and freed the people from his grasp.
After the empire's fall the different cultures which were mixed together by the forceful resettlement started to emerge again over time leading to the current political regions on Arudien. Anything linking back to the empire was destroyed and Tieflings were nearly killed off.
Not everything linked to the empire could be purged however. The empire is the reason there is one common language spoken across the continent. Tieflings, although quite rare, are also still around and no longer despised, but seen as equal members of society.