Infernal Domain

For most, a Cleric renouncing their faith is an end to their abilities as a hero. However, some people know of different ways one can receive their powers. Through great personal hardships, and the personal acquaintance of an Archdevil, a Cleric can have their powers restored by the collective energies of the Nine Hells. Such power does not come cheap, however, and a Cleric who wishes to receive such power should be willing to accept the possibility of any number of potential consequences.

"Of course, we're more than willing to assist you. Just sign here, and here, and initial here. Welcome to the family"

Divine Absence

Starting at 1st level, your abandonment of your faith leaves an empty space in your soul. When you succeed on any save of a spell which is fifth level or lower, you may as a reaction "steal" the spell, and add it to your list of domain spells known. You may learn a number of domain spells equal to your Wisdom Modifier.

Discarding Spells: If at any point you wish to remove any "stolen" spells, you may spend an action to remove all currently "stolen" domain spells. Additionally, if you have reached the maximum number of spells stolen and you attempt to steal another spell, you forget all spells currently stolen, replacing them with the new spell.

True to the powers you serve, your powers come from twisting the intents of others.

Channel Divinity: Amplify Flames

At 2nd level, you can use your Channel Divinity to amplify your flames with Hellfire.

As an bonus action, you present your holy symbol, charging it with black flames until the end of your turn. While the symbol is charged, any creatures damaged by any fire damage you deal to them are vulnerable to the damage type.

Research stolen from another world by Commander Khai, this fire burns black. While it won't hurt demons, it does cause unbelievable pain.

Infernal Finances

Your experience and position in the Infernal Bureaucracy has granted you the ability to ignore the a portion of the costs required to cast spells. When you cast a cleric spell that either requires a material component of 50gp or higher, or requires the consumption of a material component of 50gp or higher, you may instead ignore either the material requirement or the consumption of the material. In the case of a spell that requires both a material component worth more than 50gp and a consumed material component, you may choose one of the two to ignore. Once you use this ability, you may not use it again until a long rest. 

"Jewelers hate her, for this one weird trick!"

Potent Spellcasting

Starting at 8th level, you add your Wisdom modifier to the damage you deal with any cleric cantrip.

Infernal Internship

At 17th level, your service in the names of the nine hells has successfully gained you a position under an archdevil. Choose an option from the following list. You may change your infernal internship once every 30 days.

Bel, Lord of Avernus: Avernus, frontline of the Blood War and home to the greatest warriors and smiths in the nine hells. Your service and allegiance to Bel grants you resistance to Slashing, Bludgening, and Piercing damage

Dispater, Lord of Dis: A lord known for paranoia and caution, Dispater rules over the largest fortress in the nine hells, erected in case of the loss of Avernus. If Dispater is your guide, you magically know a person you can see is lying, or a written document contains clauses meant to take advantage of someone.

Mammon, Lord of Minauros: A lord known for the invention of the soul coin who rules over the central mint of the Nine Hells. If you find employment in Mammon, you gain a soul coin after the end of every long rest. You may not have more soul coins than your Cleric level on you at once from this ability.

Fierna, Lord of Phlegethos: The only female Lord of hell after Zariel's dethronement, Fierna grants her followers complete mastery over fire. Friendship with Fierna causes any fire damage you deal which is infused by your channel divinity to deal max damage.

Baalzebul, Lord of Maladomini: Cursed to a terrible form, the Slug Archduke is known most for his incredible intrigue and plots. With Baalzebul's assistance, you may spew a swarm of ravenous flies from your holy symbol. The flies protect you from harm, granting you 50 points of temporary hit points.

Asmodeus, Lord of Nessus: Lord of the Nine Hells, Asmodeus grants you the awe and respect his positon grants. With Asmodeus' powers at your disposal, you are unnable to be attacked by any creature within 120ft of you that you have not attacked first, unless they succeed at Wisdom Save, equal to your spell save DC.

"One's close associates might one day prove more adversarial than one's enemies"