The Fiend.

You have made a pact with a fiend from the lower planes of existence, a being whose aims are evil, even if you strive against those aims. Such beings desire the corruption or destruction of all things, ultimately including you.

Level 1 Features:

Fiend Magic.

When you learn a spell of the appropriate level using your pact magic feature, you also learn the spells on the Fiend Spells table. The spells count warlock spells for you, and do not count against your spells known.

Hint of Hell.

Beginning at 1st level, you can imbue one weapon you touch with a spark of infernal power. Once per short or long rest, when you touch a weapon, you can choose to make that weapon a devil arm. You are proficient with your devil arm. You can only have one devil arm and creating a new one will strip your previous devil arm of its infernal power.
If your devil arm is on the same plane of existence, you can summon your devil arm as a free action on your turn, causing it to teleport instantly to your hand.

Evil Element.

Also at 1st level, your devil arm gains a number of charges equal to your Intelligence or Charisma modifier (minimum of 1). You will regain all expended charges when you finish a long rest. You can expend a charge to do one of the following:
-Diabolic Channel: As a bonus action, you can imbue your devil arm with the arcane potential of a spell, expending the necessary spell slots. Using this bonus action will require your concentration as though you are concentrating on a spell for the next minute, or until you successfully hit a creature with your devil arm. After rolling for your devil arm's damage, the target of the attack is then subjected to the effects of the spell. Even if the spell normally affects an area or multiple creatures, it affects only the target. All spell attack rolls for the spell against the target hit automatically. If the attack was made with advantage, the targets initial saving throw against the spell has disadvantage. The spell must be of 1st level or higher and must have a casting time of one action to cast.
-Profane Dispel: As on action, you can deal 1 bludgeoning, piercing, or slashing damage (determined by your devil arms damage type) to your self, or one creature within your devil arm's reach, to end one spell affecting the damaged creature. You may cut a magical effect instead, automatically ending the effect. The level of spell you can end with this feature must be equal to or lower than half your warlock level, rounded up.
-Spell Eater: As a reaction, when you are subjected to a saving throw against a spell or magical effect to take only half damage, you instead take no damage on a successful saving throw, and only half damage on a failed saving throw. The first time you hit with your devil arm, the target of your attack will take 1d8 of the triggering spell or magical effect's damage type.

Level 6 Feature:

Icon of Sin.

At 6th level, you otherworldly manifest your infernal strength for a time. As a bonus action, you can undergo a devilish transformation to gain the following:
-Your creature type becomes fiend.
-You gain a flying speed equal to your walking speed.
-You gain 1d4 + 4 temporary hit points when you transform, and again at the start of each of your turns.
You will remain in this form for 1 minute, until you end the form early as a free action, or you die. Once you undergo this transformation, you can't do so again until you finish a long rest. When you reach 10th level, you can expend a warlock spell slot to transform again.

Level 10 Feature:

Fiendish Fortitude.

At 10th level, you have advantage on saving throws against the poison condition and resistance to fire and poison damage, or immunity while you are in your Icon of Sin.

Level 14 Feature:

Vivid Vice.

At 14th level, when you enter your Icon of Sin, two spectral copies of your devil arm will appear within 5 feet of you. When you attack a creature with your devil arm, you can also cause these spectral weapons to attack that creature, or a different creature within 5 feet of the initial target, using your devil arm's attack bonus to hit. On a hit, the spectral weapons will deal 1d10 necrotic damage. These spectral weapons will remain with you for as long as you are in your Icon of Sin.