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Let me start by freely acknowledging that the Cleric class is better-supplied with subclass options than any other class but Wizards. I expect that for a lot of people, adding an Exorcism domain is painfully extraneous. On the other hand, in my homebrewed D&D setting, I specifically need characters specialized in exorcism, and I want to go a step beyond the fact that every cleric gets the requisite spells to be a passable exorcist. Also, I've never seen The Exorcist, so as much as it pains me, this article won't be a constant stream of paraphrased quotes and ridiculous puns. God, what have I become, if I'm going punless in public?
Exorcism Domain
Gods or religious sects particularly invested in protecting mortals from demonic influence and possession may include the Exorcism domain, including Helm, Ilmater, Lathander, Torm, and Tyr in the Forgotten Realms setting; Pelor and St. Cuthbert in the Greyhawk setting; Paladine in the Dragonlance setting; Aureon, Dol Arrah, the Silver Flame, and the Path of Light in the Eberron setting; and at least some denominations of all Abrahamic faiths, Buddhism, and Hinduism in the real world (among others; I am not remotely capable of speaking broadly on this topic). In any case, the Exorcism domain is never a deity's only domain, but a small and active arm of a greater Church. In rare cases, nature deities take a strong enough position against the intrusion of creatures from Outside that they consecrate a cleric with the power to banish intruders.
As paladins and many clerics defend their faith and innocents under their care against physical threats, including warfare, pestilence, and oppression, exorcists defend against spiritual threats from other planes: celestials (particularly evil or fallen angels), elementals, fey, and fiends. These enemies may manifest in subtle ways, and an exorcist must exercise great insight to root them out without destroying innocents in the process.
Exorcism Domain Spells:
Cleric Level -- Spells
1st -- detect evil and good, protection from evil and good
3rd -- see invisibility, zone of truth
5th -- magic circle, remove curse
7th -- banishment, Mordenkainen's private sanctum
9th -- dispel evil and good, hallow
Subtle Agent of the Faith
At 1st level, you gain proficiency in two of the following: Arcana, Insight, Intimidate, any one musical instrument, the Abyssal language, the Celestial language, the Infernal language, the Sylvan language, or the Primordial language.
Armed for Spiritual Warfare
At 1st level, you either gain proficiency in heavy armor, or you gain the word of censure cantrip (see below).
Channel Divinity: Turn the Faithless
Starting at 2nd level, you can use your Channel Divinity to turn fey and fiendish creatures. As an action, you present your holy symbol and speak a prayer censuring fiends and the fey. Each creature of those types that can see or hear you within 30 feet of you must make a Wisdom saving throw. If the creature fails its saving throw, it is turned for 1 minute or until it takes any damage. If the creature's true form is concealed by an illusion, shapeshifting, or other effect, that form is revealed while it is turned. (This ability does not include destroying fiends or fey.)
Channel Divinity: Retribution against Deceit
Starting at 6th level, you can use your Channel Divinity to punish those who would invade your allies' thoughts. As a reaction, whenever an ally (not including yourself) within 30 feet fails a saving throw against the charmed or frightened conditions, or becomes possessed, you may grant that ally a new saving throw against the same saving throw DC. If the second roll is successful, you may deal 2d8 + your Charisma modifier radiant damage to the creature that forced the initial saving throw. At 11th level, the damage caused by this effect increases to 3d8 + your Charisma modifier radiant damage. At 17th level, the damage caused by this effect increases to 4d8 + your Charisma modifier radiant damage.
Potent Weapons of the Secret Crusade
Starting at 8th level, you may either you gain Divine Strike (as the 8th-level Life Domain ability), or you may add the greater of your Wisdom bonus or Charisma bonus to the damage you deal with cleric cantrips.
The Divine Interdiction
Starting at 17th level, whenever you force a celestial, elemental, fiend, or fey to make a saving throw and they roll a natural 1, or whenever you deal a critical hit to a creature of those types, it suffers a banishment effect as if cast by you. If you are currently maintaining Concentration on another effect, you can choose to drop the spell you are currently concentrating on in favor of the banishment. If you do not, the banishment lasts until the end of your next turn. For the purpose of this ability, your holy symbol is an item distasteful to that creature. (If casting banishment is what triggered this effect in the first place, you may choose a new target within 30 feet to be affected by banishment.)
Further, you gain advantage on all saving throws against the charmed or frightened effects, and against any form of possession.
Word of Censure
Abjuration cantrip
Casting Time: 1 action
Range: 30 feet
Components: V
Duration: 1 round
You deliver a stern word of rebuke, empowered by your consecration into holy mysteries, against a creature within range that can understand a language (not necessarily a language you speak). The target must succeed a Wisdom saving throw or take 1d6 psychic damage. If it ends its next turn closer to you than it began than turn, it suffers the cantrip's damage again (roll the damage a second time).
The spell's damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).
Design Notes
I did a couple of weird things in this domain, starting with the Armed for Spiritual Warfare and Potent Weapons of the Secret Crusade abilities. I didn't want to declare, as part of this subclass, that exorcists were definitely either weapon builds (for which heavy armor and Divine Strike are useful) or spellcasting builds (for which the new attack cantrip and the ability score bonus to cantrip damage are useful). Frankly, it drives me a little bananas that the Player's Handbook domains do this, and that only two out of seven support clerics inclined to hang back and cast spells. For my own campaign, I'd strongly consider a houserule to allow comparable options to all clerics, with a possible exception for the War domain, because... well, really. But a lot of deities in the back of the PH don't thematically match up with the combat style that their sole domain favors.
Subtle Agent of the Faith is, obviously, a more limited version of the Knowledge domain's first-level ability.
The inclusion of elementals in some abilities but not in others is done consciously, though I am still undecided as to whether it's the right move. My thinking is that elementals are intruders from the Inner Planes, so the Gods oppose them in some cases, but they're far less inclined toward subtle, mind-controlling malevolence. Were I running a game particularly based on Arabic and Islamic themes, I would drop fey creatures completely and grant much greater emphasis on fighting elemental creatures - specifically genies.
Remove curse is not a particularly compelling domain spell at 5th level; dispel magic might or might not be preferable.
Channel Divinity: Turn the Faithless is a shameless lift from the Oath of the Ancients paladin. It just happens that it did exactly what I wanted. Channel Divinity: Retribution Against Deceit is there because I like handing out new reaction options.
The Divine Interdiction is obviously not going to come up incredibly often, but then getting a free 4th-level spell, possibly even more than once per day, is pretty egregious. It's possible that this would be better as an active ability than as a passive proc. If it ever sees direct playtesting, I'll make a decision then. That is also why I made a point of granting the second piece of this ability - it still isn't active, but it might be more consistently helpful, assuming you're up against the kinds of creatures exorcists are specialized in fighting.
The main reason word of censure exists is that my players and I feel like clerics need a second attack cantrip option. Sacred flame is great for what it does, but an alternate option for situations when you don't care all that much about getting around cover or you really need to keep a bad guy at a distance is probably good.