Song of the Ages
Where now stand the Northern Wastes was once the great kingdom of Azahn. Within that kingdom there was a noble family that aspired for the throne. In order to achieve power and influence they made a pact with Nethos, the archfiend. With his infernal power they gained control of the kingdom, but at a terrible cost. They became disfigured, growing tails and horns, their skin became strangely colored, and they took on the appearance of their demonic patron.
Their rule lasted for a hundred years, but others within the kingdom began to thirst for the same power. Eventually greed and thirst for power consumed the kingdom, and it fell, conquered by holy crusaders from the south.
Some of the descendants of that noble family survived the fall of Azahn, and have since become lost wanderers. Today, though rare, the remainders of Azahn are known as Tieflings.
The appearance of tieflings reflect their demonic heritage, with skins colors varying from red and purple to ash or white. They have horns and tails, hooved feet, sharper than usual teeth, and inborn magic born of the fiends.
Tieflings are not evil by nature, but the distrust and mistreatment of others causes many born with the curse to take the path of darkness out of necessity or spite.