Occultist

An occultist is a cleric of a creature from lower planes. Someone who dedicates his whole life to serve a demon, or a devil, as a priest.

Bonus Proficiency

When the cleric chooses this domain at 1st level, he or she gains proficiency with martial weapons, heavy armor and in deception.

Fiendish Flame

Starting at 1st level, whenever you use a spell of 1st level or higher to deal fire damage, the creature loses additional HP equal to 1d4 + the spell’s level.

Additionaly every spells that normally deals radiant damage, deals fire damage

Channel Divinity: Drain Life

Starting at 2nd level, you can use your Channel Divinity to reap health from your enemies.

As an action, you present your holy symbol and evoke necrotic energy that can take away a number of hit points equal to five times your cleric level. Choose any creatures within 30 feet of you, and divide those hit points among them. This feature also heals the user up to half of the damage dealt. You can’t use this feature on an undead or a construct.

Acolyte of the Hells

Starting at 6th level, You gain resistance to fire damage and poison damage.

Fiendish Familiar

Beginning at 6th level, you know how to conjure fiendish companions to aid you. You may summon a familiar as if casting the find familiar spell. This creature's type is considered fiend and has resistance to poison and fire damage, and follows the rules of the find familiar spell.

Fiendish Strike

At 8th level, the cleric gains the ability to infuse his or her weapon strikes with fiendish power. Once on each of the cleric’s turns when he or she hits a creature with a weapon attack, the cleric can cause the attack to deal an extra 1d8 fire damage to the target. When the cleric reaches 14th level, the extra damage increases to 2d8.

Mastery of Fire

At level 17, As an experienced fire user, you can roll max damage for all fire damage. This can only be done once per short rest. You activate this ability before making any attacks, and it lasts your entire turn. This means casting a level 3 fireball will inflict 60 fire damage (8d6 + 3d4 from Disciple of Damage) on a failed save, and 30 damage on a successful save. You also gain the control flames cantrip.