Mommy, why am I a Tiefling?
A Quora user asked the question: In DND, can a Tiefling be born from two human parents? My answer to any question along these lines is almost always going to be: Yes! (setting norms permitting) To find out why mummy and daddy are human and you're not, pick or roll 1d20: Thauma-genetic manipulation at the fertility clinic. Surrogate birth. Cuckoo demon left an extra bun in their oven while no one was looking. Your twin sibling looks perfectly human though. Parents made a pact with the Infernal powers, dedicating their next child to the Grand & Decadent Cause. (ANGSTY TEEN TIEFLING: Jeez! Lay off mom and dad! I don't wanna do your stupid dark destiny plans!) One of your parents used to be an infernal pact warlock before you were born. They turned from that path a long time ago, repented, and tried to make amends for the things they did during those times. But a part of the pact they'd overlooked ensured their first born would be a Tiefling. So here you are...