Tieflings are the result of a pact struck generations ago by the people of the city of Shale and Chhinnamasta — a ferocious aspect of the Mother goddess. The self-decapitated nude goddess, usually standing on a divine copulating couple, holds her own severed head in one hand, a scimitar in another. Three jets of blood spurt out of her bleeding neck and are drunk by her severed head and two attendants.
Flint was besieging the city and they struck a deal with the goddess - she would save the city and in return she would infuse their nature with her own demonic bloodline.
The Tieflings appearance and their nature are not their fault but the result of this ancient sin, for which they and their children and their children’s children will always be held accountable.
Tieflings are derived from human bloodlines, and in the broadest possible sense, they still look human. However, their infernal heritage has left a clear imprint on their appearance. Tieflings have large horns that take any of a variety of shapes: some have curling horns like a ram, others have straight and tall horns like a gazelle’s, and some spiral upward like an antelopes’ horns. They have thick tails, four to five feet long, which lash or coil around their legs when they get upset or nervous. Their canine teeth are sharply pointed, and their eyes are solid colors—black, red, white, silver, or gold—with no visible sclera or pupil. Their skin tones cover the full range of human coloration, but also include various shades of red. Their hair, cascading down from behind their horns, is usually dark, from black or brown to dark red, blue, or purple.