You or your ancestors signed a contract with a devil, and now your body changes as a result of it and gains fiendish features.
Ability Score Improvement. Increase one ability score by
1, to a maximum of 20.
Devil's Sight. Magical darkness doesn't impede your vison and darkvision.
Fiendish Evolution. Your creature type changes to Fiend.
Further, you acquire a physical disfigurement as a hideous sign of your fiendish evolution. An evil rune might appear on your face, your eyes might become glossy black, horns might sprout from your forehead, or some other feature the GM chooses.
Magic Resistance. You have advantage on saving throws against spells and other magical effects.