Fiendish Domain

You have not chosen a divine god to pray to, rather a demonic force. This can be a demonic god, fiendish beholder, or other high powered demonic force. By praying to these, you gain there fiery power, but (optionally) you also slowly transform into a fiend. This subclass focuses on fire damage.

Fiendish Radiance

Any radiant damage you do from a spell or attack is replaced by fire damage. If you create any magical objects or enchantments that would have radiant effect, they now have fire effects. Radiant enchanted weapons will still do radiant damage.

Deciple of Damage

Whenever you cast a spell that inflicts damage, it does an extra 1d4 fire damage for each spell level to each target. For example, a level 1 multishot attack targeted at one enemy will only deal one extra 1d4 fire, but a level 1 area of effect attack will do an extra 1d4 fire to each target. If the spell has multiple shots, then the damage is added to the first one that hits. The extra damage is rolled once and applied to all targets.

Channel Divinity: Turn Holy

At level 2, You gain the ability to let out a demonic shriek, and all creatures that can hear you in a 30 ft. radius (including party members) that aren't undead or fiendish have to make wisdom saving throws. If they fail, they are turned for one minute or until they take damage.

A turned creature must spend its turns trying to move as far away from you as it can, and it can't willingly move to a space within 30 feet of you. It also can't take reactions. For its action, it can use only the Dash action or try to escape from an effect that prevents it from moving. If there's nowhere to move, the creature can use the Dodge action.

IMPORTANT: This replaces turn undead, so you are no longer able to use turn undead. Also, destroy undead works against plant creatures instead of undead.

Channel Divinity: Spontaneous Combustion

At level 2, by channeling the power of your demonic deity, you explode in a fiery ball of death. All creatures in a 20 ft. radius must make a dexterity saving throw, with disadvantage. You automatically fail the saving throw, and take full damage (even if you have resistance to fire damage). Creatures that fail the check take 2d8 fire damage. If they succeed, they take half damage. However, this cannot outright kill you, even if brings you to negative max hp. Spells that would prevent unconsciousness instead only stabilize you if you reach 0 hitpoints.

Every two levels above 2nd increase the damage by 1d8. A level 20 fiendish cleric deals 11d8 fire damage.

Fiery Blood

At level 6, Your veins course with the fire you worship. Whenever you take slashing or piercing damage (magical or non-magical), you deal 1d6 fire damage to all creatures within 5 feet, and an additional 1d6 for every 10 damage taken above 10. For example, a critical crossbow shot that deals 20 damage to you will cause 2d6 fire damage to all creatures within 5 feet, including allies.

Fiendish Transformation

[optionally] (does not replace anything, just an interesting addition with minor improvements to make worthwhile)

At level 6, your connection with the demonic force begins to transform you closer to a tiefling. Your skin becomes a slight pink shade, your entire eyes are tinted a solid color over their usual appearance, and you grow a small thin tail that can be hidden. This happens while you are asleep, and you only realize once you wake up. Npcs don't immediately recognize you as a tiefling, but anyone who spends a long time around you (including the party) will be able to tell.

At level 10, Your skin becomes a medium-light shade of red or purple, and your eyes become fully black, red, white, silver, or golden, with no pupil or iris. Small horns start to sprout from your head, and your tail grows to 2 ft. with moderate thickness. This also happens during a long rest. Npcs will assume you to be a tiefling. You gain darkvision for 60ft. if you don't already have it. 

At level 14, Your skin is a deep shade of red or purple, and your horns fully grow out. Your tail grows to 4-5 ft. and is thick. You gain resistance to fire damage. You keep the size, stature, features, and abilities of your original form, but you are now considered to be tiefling by Npcs.

Channel Divinity: Overcharged Combustion

At level 8, if you have 2 or more Channel Divinity charges, as an action you can Overcharge and explode, using up all Channel Divinity charges. You create an instantaneous explosion in a 30ft. radius around you. All creatures within it take 8d8 fire damage, with an additional 1d8 fire damage for every cleric level above 8th. You take full damage (regardless of resistances or protecting spells), and can instantly die if you reach negative max hp. When you use this, you are knocked unconscious for 2d4 hours & get a point of exhaustion. If this brings you to 0 hp, you start with one death save failed. Spells can prevent you from reaching 0 hp, but you are still knocked out for 2d4 hours. You always get the point of exhaustion.

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.