Your Strength, Dexterity, Wisdom, and Charisma scores all increase by 5, to a maximum of 20 for each.

Magical darkness doesn't impede the devil's darkvision.

Your ancient heritage grants you inherent understanding of infernal, as well as two other languages of your choice.

As an action, you can transform your appearance or revert to your natural form. You revert to your natural form if you die.

You decide what you look like, including your height, weight, facial features, the sound of your voice, coloration, hair length, sex, and any other distinguishing characteristics. You can make yourself appear as a member of another race, though none of your game statistics change. You also decide if your clothing and other equipment also change in appearance. You decide what your clothing and equipment look like. size, shape, material, colour, and any accessories are made up from your equipment.

Even to the most astute observers, your ruse is usually indiscernible. If you rouse suspicion, or if a wary creature suspects something is amiss, you have advantage on any Charisma (Deception) check you make to avoid detection.

As an Archdevil, you resist necrotic, and cold damage.
immune to fire & poison.

you have the ability to step from one shadow into another. When you are in dim light or darkness, as a bonus action, you can magically teleport up to 120 feet to an unoccupied space you can see that is also in dim light or darkness

If a devil dies somewhere other than the Nine Hells, the body disappears with a flash of fire, a puff of smoke or dissolves into a pool of ichor and re-forms in the Nine Hells, it is therefore not truly killed. However, a devil killed in the Nine Hells is dead forever; not surprisingly, that’s why devils tend to be more cautious in their dealings at home than abroad.