While Tieflings are descended from devils you are descended from a fiend.

Hidden in Plain Sight

You are able to transform yourself into other races or humanoid creatures that you have seen.  You keep your relative features and size such that someone whom knows you might mistake you for a relative or recognize you if they are aware of your ability.

Your transformation only slightly alters the shape of your body, it does not alter your hit points, attributes, skills, or abilities.  Any equipment you are wearing or carrying isn't transformed, and you revert to your true form if you fall unconscious or if you die.

You must be able to perform a type of movement to use it in your changed form, but if you can then you have your full movement speed if that movement type is natural to your changed form.  You can transform a number of times per short rest equal to your Charisma modifier and can hold a particular form as long as you are conscious.  This is a natural ability, unlike Disguise Self or Alter Self it is not a spell.

You have darkvision  up to 60 feet.

Impish

At level 6 your teeth grow sharp, you form claws, a prehensile tale, and wings.  You also sprout small horns and your skin turns an unnatural color, such as bight red or black... and you thought Tieflings had it rough.  This is now your true form when you are not transformed.

You can use your claws for unarmed attack with STR or DEX.  You can use your tail for an attack, tripping passers by, or to hold and manipulate objects in a similar manner to Mage Hand (if it were attached to your back).  Your wings allow you to fly at a speed of 40 feet, taking off and landing requires a bonus action.  You can even headbutt using your horns.

As long as you are in a form that has claws (or nails), horns, or a tail then you are able to use the attacks specific to the anatomy available.  You can only fly if you are in a form that has wings, and you can only use your tail to manipulate objects if you are in a form with a long enough tail.

Regardless of your appearance, due to your true form, you are now considered a fiend for purposes of detection and spell targeting.

Dark Blessing

At level 14 you may select two options from the Dark Blessings list.

Fiendish Aura

Once per short rest, any creatures you choose within 30 feet of you must make a WIS save versus your spellcasting ability or else be frightened of you for 1 minute.  If you are invisible, and they cannot see invisibility, then they are instead stunned for 1 round.

Fiendish Blessing

Your unarmored armor class is 10 plus your Dexterity modifier plus your spellcasting modifier, Charisma.

Fiendish Charm

Once per short rest you can cast Charm Person without components.

Fiendish Vigor

You can cast False Life on yourself at will without components as a Bonus Action.

Fiendish Weapons

When you attack with your natural abilities then you may add 1d4 + your spellcasting ability fire damage.

Abyssal Resistance

At level 18 you gain immunity to Fire and Poison damage, and the Poisoned condition.  You also gain resistance to Cold damage and Bludgeoning, Piercing, and Slashing from Nonmagical Weapons.