You have darkvision. You can see for 80ft in darkness as if it were bright light, and another 80ft as if it were dim light. Anything beyond that comes as vague shapes

you have flying speed of up to 40ft per turn, + 10 flight speed the larger you are and - 10 flight speed the smaller you are.

You can spend your action to look directly at a creature that isn't behind total cover and attempt to frighten them with your gaze. They must roll a Constitution saving throw against 10 + your Charisma modifier. On a success, they are frightened for 1 round + 1 round for every character level you posses, and if they fail they are paralyzed for the same duration.

You can use your action to summon and throw a small ball of fire in your hand to use as an attack. It has a range of 15ft - 30ft, with a 5ft increase or decrease in range depending on your size, ratioed from the medium size. The base damage is 1d6 per character level, modified by your Charisma score, and has an AOE of 5ft around the intended target. This ability has 5 charges, and each use takes one charge. You can regain 1 charge after every short rest and all of the charges after every long rest. You gain 5 more charges onto the maximum at every fifth level.

You have the ability to enchant the weapons of like-aligned owners with the power of fire or death. This ability has 3 charges, which can be regained after every long rest. You can spend one charge to make a weapon a flaming version of itself for a temporary duration, or two charges to make the weapon a +1 version of itself for 24 hours plus your level in hours. The damage gained by the enchantment scales with your level.

Flaming Blade- Level 1- 1d4 bonus fire damage. Level 5- 2d4 bonus fire damage. Level 9- 3d4 bonus fire damage. Level 13- 5d6 bonus fire damage. Level 17- 7d6 bonus fire damage. Level 20- 9d10 bonus fire damage

Screaming Blade- Level 1- 1d4 bonus cold damage. Level 5- 2d4 bonus cold damage. Level 9- 3d4 bonus cold damage. Level 13- 5d6 bonus cold damage. Level 17- 7d6 bonus cold damage. Level 20- 9d10 bonus cold damage

Blade of Death- Level 1- +1. Level 5- +2, Level 9- +3, Level 13- +5, Level 17- +7, Level 20- +10

If this ability is used on the same weapon once every week for one year, then the effect becomes permanent

Your Charisma score increases by 3 and your Strength score increases by 2

You summon a glowing orange sword to your hand with the following properties: Versatile, Finesse, Thrown 20/60. The sword is summoned as an action, but dispelling it is a bonus action. If the sword gets knocked out of your hand, you can use your reaction to call it back to you. If the sword is out of your hand for more than 1 round, it disappears. The sword acts as a longsword, but does 2d8 or 2d10 damage thanks to the versatile property. The damage increases to 3d8 or 3d10 at level 5 and the sword becomes a great sword. At level 9, the damage increases to 4d8 and 4d10 and the sword becomes a bastard sword, and from then on it remains a bastard sword. At levels 13, 17, and 20, the damage improves by one die for each score respectively.

As an action, you can choose to improve your physical capabilities for an amount of turns equal to 1 + your level. During this time, your Strength score becomes 30, your armor class becomes 25, and you gain 10x your level in temporary hp. Your jump distance changes to 200ft, your climb speed changes to 150ft, your movement speed changes to 250ft, you can take the dash action as a bonus action and reaction. This ability has 1 charge at level 1, 2 charges at level 10, and 3 charges at level 20 and costs 1 charge to use. You regain all spent charges after a long rest.

This ability has 6 charges, and each option costs a different amount of charges to use. The duration of the chosen feature is equal to 10 rounds plus your level.  All expended charges are regained after a long rest, or 2 expended charges are regained after a short rest.

Blindsight- 1 charge

Tremorsense- 2 charges

Truesight- 3 charges

As an action, you can transform your fingers into flaming claws. Your unarmed attack damage becomes 1d4 plus your Strength modifier, plus 1d4 fire damage. The fire damage goes up by 1d4 every four levels (Levels 5, 9, 13, 17, 20)

You gain resistance to acid, force, thunder and  non-magical bludgeoning, slashing, and piercing damage, and you gain immunity to fire, cold, psychic, necrotic, poison damage. You cannot be poisoned or diseased. You also gain vulnerability to radiant and lightning damage.

You gain advantage on saving throws and rolls made to defend against spells meant to posses or control you, and you become immune to the charmed condition.

This ability has 3 charges and costs one charge to use. When used, choose a target. That target must make a Wisdom saving throw with disadvantage against a DC of 10+ your Charisma modifier and your Strength modifier. On a success, the target becomes terrified of you for 5 rounds plus 1 round per your level. On a failure the target becomes paralyzed with fear for the same duration.

You gain immunity to the frightened condition, and can't be affected in any way by any ability pertaining to fear (Including those of other devils)

Once every ten days, you may offer a bargain to any mortal you choose. A boon from among those listed, in exchange for their temporary service and whatever else you wish to make them pay. They serve out their sentence first (8 days+ 2 days/ level) before receiving their boon(s), which last for 1d4 weeks +1/level (Boons become permanent when archfiend reaches level 20). During their "Service", their soul is bound to your own and they become helpless slaves to your being for the entire duration, and nothing shot of a wish spell will break this effect (Although you may choose whether or not they still receive their boon if they are freed in this manner). Any damage you take is split evenly among you and any currently serving you (You can have a maximum of 5 recipients bound to you at any one time) and they can be killed in this manner. Any damage you receive will be spread evenly among recipients until they have reached a number below 0hp equal to their normal maximum hp, at which point their souls will die and they cannot be resurrected short of a wish spell. A mortal can take multiple boons from your list, but if they do so it extends the duration of their service by 2 days/ level of you. You cannot grant these boons to yourself, and the recipient of the pact must be willing (Ie, not under the effects of a suggestion or charm spell, and not forced by the archfiend)

Available boons:

+5 to any ability score (Recipient's choice), Advantage on everything, 1d6x 100 000 gp, the free automatic casting of one wish spell (expires in 1 year), a bonus to weapon damage and accuracy (Max of +10 each), One additional spell slot of every level for your casting class, one extra attack action, or the sneak attack feature

Boons at level 20:

+10 to any 2 ability scores (recipient's choice), Advantage on everything (Permanent), 6d6x 100 000 gp, the free automatic casting of 3 wish spells (Doesn't expire), a bonus to weapon damage and accuracy (Max of +50 each), three additional spell slots of every level for any casting class, three extra attack actions, the sneak attack feature, the ability to attack your attacker as a reaction, or the ability to cast a number of spells at once equal to your spell modifier. 

At will, you can cast an illusion over yourself to make you appear to be of any race besides your own. Your illusion is flawless, except for your eyes, which always take on a red coloration. Your illusion can't be detected through any normal means. Only magical forms of detection, such as detect magic, can detect it if they succeed against a DC of 12+ your Charisma modifier and your Wisdom modifier. A wish spell will automatically reveal your identity to the caster of the wish spell. While you are in your concealed form, you still have access to all of your fiendish traits.

Your blood will melt through metal at a rate of 6 inches/ turn, starting on contact, and will melt through stone at a rate of 1 foot per turn. If left on flesh, wood, mud, or clay, it burns through it at a rate of 2 feet per turn starting on contact. Your blood won't melt through anything under the effects of a Bless spell, or anything under the effects of a Hallow spell. Consecrated ground and holy objects have the same effect, and by pouring holy water onto the blood it counters the acidic properties of it. Work with your DM to determine the rate at which your blood melts through any substance not listed here. 

Choose one creature you can see as your target, that creature rolls a Wisdom saving throw with disadvantage against a DC of 10+ your Charisma modifier and your Wisdom Modifier. On a success, they repel your illusions but become dazed and lose their next turn. On a success, they become incapacitated for 1 turn/ your level with visions of your time under the torture of your master, and suffer 1d4 psychic damage per turn while incapacitated. At the end of the effect's duration, the target must roll on the long term madness table to see what the result is. They gain that result for the full duration.

At level 10- the duration becomes 2 turns/ your level, and you can choose an additional target for this ability that you can see

At level 20- the duration of the madness effects becomes 1 year, and you can choose a third target that you can see for this ability.

You automatically add one suit of Hell Plate armor to your inventory upon character creation.

Hell Plate Armor- The armor is always custom-fit to the wearer, and it is a set of overlapping plates designed to provide maximum protection with maximum mobility. While wearing the armor your armor class is 15+ your Dexterity modifier. While wearing the armor, you gain a +5 bonus to any Dexterity roll made and your armor class, and you can add your Dexterity bonus to any weapon or attack roll made (Along with your Strength bonus)

At higher levels, the devil gains the passive ability to project its aura to the world around it.  Starting at level five, your footsteps burn into the material you walk on, though they don't set anything ablaze. At level ten, an area equal to a five foot circle around you becomes burnt and dead, as if you'd walked across it all. At level fifteen the circle becomes twenty feet, and at level twenty the circle becomes eighty feet and the results of your passing are permanent, and can only be undone if placed under a bless or hallow spell and then magically removed. 

Fire heals you. From 1st to 4th level, you regain health equal to half of the damage you would've taken from fire. From 5th to 9th level, you regain health equal to the damage they would've done. From 10th level to 14th, you regain health equal to twice the damage they would've dealt, and from 15th to 20th level, you regain health equal to four times the amount of damage they would've dealt. This healing also stacks as temporary extra hit points, that last until you complete a long rest. 

You can summon four demons from the abyss to serve your every need for exactly 24 hours after their summoning unless they are slain or banished beforehand. The challenge rating of the demons you can summon will be equal to your level. 

Using your action, you can change your size to either one size smaller or larger than your current size. This affect lasts until dispelled by yourself has one charge per day, and each charge is regained at every midnight.