Tieflings are the cursed (or blessed, depending on your point of view) descendants of a group of humans that were lured into the Underworld by the Sultan of Horror and the God of Malice. These dark powers twisted and corrupted them, granting them a domain to rule but secretly grooming them to overthrow the gods. Eventually, the God of Death learnt of the evil God's actions, and offered their chosen a choice: those who wished to remain as their were would be trapped within the darkness of the Underworld, but the others could return to the surface at the cost of their new powers. The descendants of those who returned became Tieflings, the taint of the evil gods remaining upon them. Those who stayed became the first Demons, unable to leave their part of the Underworld without outside intervention.
Tieflings are generally mistrusted by the people of the Archworld, even by those few who understand their heritage. A few, however, manage to find acceptance through their actions. Others become among the land's worst villains.
(Tieflings use the standard D&D 4th Edition Tiefling stats, found on page 48 of the Player's Handbook.)