Infernal Domain

Instead of serving a god, Clerics of the Infernal Domain swear worship to an Archdevil or Archdemon, such as Dispater or Asmodeus.

Channel Unholiness: Unholy Wrath

Starting at 2nd level, you can use your Channel Divinity to wield the infernal power of your Fiend with hellish might.
When you roll fire damage, you can use your Channel Divinity to deal maximum damage instead of rolling.

Hellfire

Starting at 6th level, your hellish power creates everlasting flames. Whenever you roll fire damage, and any die rolls its maximum value, the attack leaves the target with a hellish burn that continues to blaze after the attack. At the start of each of its turns, a creature affected by this feature takes damage equal to 1d4 + your Wisdom modifier until they use their action to douse the flames.

Hellish Strike

Starting at 8th level, you gain the ability to infuse unholy power into your weapon strikes. Once on each of your turns, when you hit a creature with a weapon attack, you can cause the attack to deal an extra 1d8 fire damage to the target.
When you reach 14th level, the extra damage increases to 2d8.

Blackened Visage

At 17th level, you gain resistance to fire, necrotic, cold, and radiant damage. Additionally, you can use your reaction to impose disadvantage on attacks made against you by a celestial- this ability can be used a number of times equal to your Wisdom modifier (minimum of once), and regain all expended uses after a long rest.

Bonus Proficiencies

When you choose this Domain at 1st level, you gain proficiency in martial weapons and the Deception skill.

Dark One's Blessing

Also at 1st level, when you reduce a hostile creature to 0 hit points, you gain temporary hit points equal to your Wisdom modifier + your cleric level (minimum of 1).

Channel Unholiness: Turn the Holy

Because of the nature of the being you serve, instead of channeling divine power like most Clerics, you channel unholy energy.
Your Channel Divinity feature becomes Channel Unholiness, which functions the same way. Instead of using Turn Undead, your Channel Unholiness has the Turn the Holy effect.
Turn the Holy behaves the same way as Turn Undead, but instead of affecting undead it affects fey and celestials.

When you gain the Destroy Undead feature, this feature affects fey and celestials in the same manner, using the same table.