An Ancient Domain cleric may worship an ancient deity such as Tharizdun or something far more ancient. They have eldritch powers and are often considered cultists by most members of society. While they are outcast from society, often the gods they worship may not be evil, they have obscure motives and are not understood by most. 

CLERIC   Level	Feature
1st	Domain Spells, Forbidden Knowledge
2nd	Channel Divinity: Eldritch Corruption
6th	Mind Fracture
8th	Potent Spellcasting
17th	Unknowable Power

Domain Spells

 

Cleric Level	Spells
1st	Arms of Hadar, Charm Person
3rd	Hold Person, Ray of Enfeeblement
5th	Fear, Hunger of Hadar
7th	Blight, Shadow of Moil
9th	Geas, Negative Energy Flood

Forbidden Knowledge

When you chose this subclass at first level you gain 2 cantrips from the warlock spell list. These count as cleric cantrips for you. 

Channel Divinity: Eldritch Corruption

Starting at 2nd level you can use your channel divinity to infect the mind of a creature. 

As an action you present your holy symbol, and one creature within 30 feet of you must make an intelligence saving throw, if the creature fails the saving throw it takes 2d8 psychic damage and is stunned until the end of its next turn. 

Mind Fracture

At 6th level the Eldritch nature in which you cast your spells is so alien it damages the minds of those who watch you. Whenever you cast a spell of 1st level or higher it deals 1d8 psychic damage to one creature of your choice that you can see within 30 feet of you and you regain hit points equal to half the damage dealt. 

Potent Spellcasting

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

Unknowable Power

At 17th level, the cosmic power granted to you by your deity has grown so powerful you can release it all in one huge burst. As an action, you present your holy symbol and expend up to 12 levels worth of spell slots, each creature within 30 feet of you (including yourself) must make a constitution saving throw against your spell save DC, taking 1d12 × the amount of spell slots expended force damage (for example if you expended two 1st level spells slots and one 3rd level spell slot a creature that failed the save would take 5d12 force damage) or half as much damage on a successful save. Afterwards you gain 1 level of exhaustion and cannot use this feature again until you finish a long rest.