Ad Astra Per Arcana
You are a tiefling. You might be an escaped slave, a hidden shame, or a homeless vagabond, but whatever your upbringing, life has been particularly hard on you. You have suffered greatly, nearly starving to death one winter, nearly being beaten to death by racist sailors one summer, and so on. Whether or not these experiences have made you a bitter and cynical scoundrel or a pious and hopeful optimist is up to you, but one thing is certain—you are something less than even your tiefling kin. Perhaps it is a result of your hard life, or perhaps it is due to some fault in your fiendish heritage, but you lack a tiefling’s standard resistances to cold, electricity, and fire—instead, you merely have a +2 bonus on all saving throws made against these effects. Likewise, you do not have the ability to use darkness as a spell-like ability once per day—instead, you may choose any one 0-level spell that you can instead use at will as a spell-like ability.
Council of Thieves Player’s Guide