Eldritch invocation: Infernal Combustion

Your connection with your hellish patron allows you to wield powerful magic, because of this your soul is connected to the nine hells and luckily for you It’s a two-way street. When you use this spell, you open your soul and body to the hells, your soul passes through each layer of the Hells, gaining their essence and their torment, allowing you to assume the form of a devil temporarily, you are considered a fiend while in this form. Note that this ability may be used an amount of times equal to your proficiency bonus(maximum of 3), your usages are restored after a long rest.

At Level 5 You Gain

Devil Form: On your turn, you can tap into demonic energies as an action and enter a new form. This form lasts for 5 minutes or can be dispelled with an action, you gain the following benefits:

• Demonic Biology: You have a +2 bonus to AC and gain 50 temporary hit points. If any of these remain when the ability ends, they are lost. You also have immunity to fire damage.
• Devil Breaker: You may add your Charisma modifier to all damage rolls(minimum 1).
• Devilish Movement: Your movement speed increases by 15 feet.

If you start your turn with fewer hit points than half your hit point maximum, you must succeed on a DC 15 Constitution or revert back to your original form. Immediately after the ability ends, you must succeed on a DC 15 Constitution saving throw or suffer two levels of exhaustion for the duration equal to how long you were in Devil Form(maximum of 5 minutes) and you take 1d20 force damage.

At Level 7 You Gain

Improved Devil Form: Your usage of this form has allowed you to gain a deeper knowledge of how your demonic powers work, allowing you to add the following benefits when you enter Devil form:

• Improved Devil Breaker: You can Attack twice, instead of once, whenever you take the Attack action on your turn. You ignore this benefit if you already have a feature, like Extra Attack, that gives you extra attacks.
• Devil Arm: You gained the ability to manifest a weapon made of your demonic energy, As a bonus action you may summon a Hell Chain whip(1d8 magical slashing damage) and Instead of adding your Dexterity or Strength Modifier to the chain, you add your Charisma Modifier to Attack Rolls. The chains extends 30 feet. You are proficient with this weapon and you have full control over this chain, allowing you to manipulate its movement, you may use another bonus action to dismiss the Hell Chain. As a result of focusing your power into a physical form, you are not able to cast spells while wielding the Hell Chain.
• Improved Demonic Biology: You are resistant to piercing, slashing, and bludgeoning damage from non magical sources.

At Level 12 You Gain

Archdevil Form: Your body has strengthened it’s resistance to your demonic form, allowing you to safely overflow your body with the full power of the Hells, also while you are in your Devil form you can use an action to switch to Archdevil form. This form lasts for 1 minute and you gain the following benefits:

• Devil Breaker: You may add your Charisma modifier to all attack and damage rolls(minimum 1). as well as you can Attack twice, instead of once, whenever you take the Attack action on your turn. You ignore this benefit if you already have a feature, like Extra Attack, that gives you extra attacks. Moreover, you can cast one of your cantrips in place of one of those attacks.
• Demonic Biology: You have a +5 bonus to AC(maximum of 25) and gain 60 temporary hit points. If any of these remain when the spell ends, they are lost. You have immunity to all nonmagical physical damage, and resistance to all magical physical damage. At the start of each of your turns, you regain hit points equal to your charisma modifier(minimum of 1).
• Hellbent: You gain Truesight 60 feet.
• Devilish Movement: Your movement speed increases by 20 feet and flying speed of 60 feet.
• Devil Arm: Your unarmed attacks now do 2d10 magical slashing damage. 
• Fear The Devil: All creatures within 30 feet of you when you activate your Archdevil form, must succeed on a wisdom saving throw with DC equal to 8 + your charisma modifier + your proficiency bonus, or become frightened of you for the next minute. A creature may repeat this saving throw at the start of each of their turns. A creature who succeeds on this saving throw is immune to this effect for the next 24 hours.

While in Archdevil form, you do not gain the benefits of the Devil form. Immediately after the spell ends, you suffer 5 levels of exhaustion for 1 minute.