Coming out of the dangerous heat of Death Valley, the post-humans known as the Tieflings are viewed with much suspicion by the other races. According to legend, they survived the Crap Times by making an evil pact with an otherworldly power, and those who claim to this legend point to their horns, tails, and solid-color eyes as proof of the claim on their souls.
But the Tieflings themselves swear that (at least as far as they know) they were just changed by the fallout same as everyone else. They claim that their preternatural resistance to heat and flame, and their innate magical ability to summon a cloud of darkness, are merely adaptations spurned by the unpredictable wild magic to enable them to survive in the harsh terrain where they took shelter from the bombs.
Individual Tieflings deal with this prejudice in their own ways. Some choose to live up to the worst that is said about them, while others try to overcompensate in the other direction. Some take advantage of the suspicion their heritage draws to intimidate, and others make themselves as invisible as possible. On the whole, Tieflings don't enjoy working with others due to the baggage that must be overcome, but some individuals are eager to prove that the legends are false, and what better way than collaboration and camaraderie?