Using a form of Alter self, you can shift between a normal seeming human form, and a demonic form that has some additional abilities
Horrific Visage- Advantage on Intimidation checks
Scales- when you are not wearing any armor, your base armor class becomes 13
Wings- Your shoulders sprout batlike wings that give you a fly speed of 60
multi-attack- you gain formidable Claws and a vicious bite.  as a single attack action you can attack with one claw and a bite at your base attack bonus  These count as natural weapons, and as magical for the purpose of bypassing resistance.

Note that while in human form you do not have access to these abilities

Your strength, dexterity and Intelligence scores increase by 4 (maximum of 20), your Constitution and Charisma scores both increase by 2. (maximum of 20)

Half Fiends are always evil, being only one generation removed from their fiendish parent-

Half fiends are the same size as their non-fiendish parent.   Your size is Medium.

Your base walking speed is 30 feet.  Your fiendish form has a fly speed of 60 feet.

You also have Devil's Sight as per the Warlock invocation- allowing you to see in any sort of darkness, including magical darkness,  as if in full daylight.

You have resistance to fire, cold, acid, and lightning damage. Resistance to non-magical weapon damage, and advantage on Saving throws against Spells

As you progress in levels, you gain access to more innate spells that you can cast without needing components or expending a spell slot.

• Level 1- you know the Thaumaturgy and Fire bolt cantrips, and can use Alter Self at will, but only between your human and demonic form. You can also use Hellish rebuke 1/long rest.
• Level 3-  Darkness 3/long rest
• Level 5-  Fireball 1/long rest
• Level 7- Fire Shield 1/long rest
• Level 9- immolation 1/long rest
• Level 11- Harm
• Level 13- Divine Word 1/long rest  (Profane word)
• Level 15- Teleport 1/long rest
• Level 17- Glibness 1/long rest
• Level 19- Gate 1/long rest

Charisma is your spellcasting ability for these spells

You can speak, read, and write Common, Abyssal, and Infernal.