A cynic might say that the only difference between an “acolyte” and a “cultist” is one’s point of view, but the Monster Manual would disagree. Piety takes different forms depending on whether an NPC is an extra or an antagonist.
The fifth-edition MM doesn’t even do cultists the courtesy of giving them spells to cast. In fact, for the most part, they’re just shifty commoners with exotic swords. They’re not strong, they’re not tough, and they’re not even stealthy. They have proficiency in Deception, whose only function as far as I can tell is either to try to convince you that they’re not cultists at all (“Nope, no sirree, just ordinary villagers going about our ordinary business!”) or for recruitment purposes (“Say, how’d you like to come to our low-key get-together/self-actualization workshop/poetry reading?”). Dark Devotion is an interesting feature but not one that suggests any distinctive combat tactic.
Normally, a creature whose only above-average ability is Dexterity is some sort of ranged sniper, but cultists don’t even carry ranged weapons. And why on earth would a demon worshiper need to have slightly above-average Dexterity? What is their actual deal?
It seems to me that the only useful way to think of cultists is as commoners with unusually intense personalities. What does this mean? Well, for starters, their first and foremost priority is not to be discovered as cultists. If the player characters encounter them socially, they’ll do all they can to reveal their loyalties. Only if their cover is blown will they either fight or flee—the former if they outnumber the player characters, the latter if they’re the ones outnumbered. They’ll also fight to the death, knowing that they could be executed for their heresies, but also believing that infidels such as the PCs deserve to die themselves. Aside from that, they fight with as little sophistication as any other commoner.
Cult fanatics, on the other hand, are no joke, mainly because of their spellcasting kit, which includes spiritual weapon and inflict wounds. Spiritual weapon is cast as a bonus action and continues to provide additional bonus actions over the next nine rounds, making it an indispensable component of the cult fanatic’s action economy. Inflict wounds is a potent melee spell attack that does necrotic damage on a hit (in our group, we’ve nicknamed it “the Bad Touch”). Compare it with the cult fanatic’s melee Multiattack, which includes two dagger strikes for a potential total of 2d4 + 4 piercing damage (9 hp, on average): cast using a 1st-level spell slot, inflict wounds does 3d10 necrotic damage (16.5 hp, on average), and boosted to 2nd level, it does 4d10 (22 hp). That’s serious.
Other spells the cult fanatic can cast include hold person (Wisdom save, causes paralysis, requires concentration), command (can provoke opportunity attacks by causing melee opponents to flee), shield of faith (bonus action, requires concentration) and sacred flame (does 1d8 radiant damage from a distance). All in all, a potent kit for a level 4 spellcaster.
As NPCs with high Dexterity (still mystified by that) and slightly above-average Constitution, the cult fanatic has two plausible modes of combat: ranged sniping and scrappy melee fighting. In either case, having numbers is very important—either other cult fanatics or rank-and-file cultists who can divide the PCs’ attention. If accompanied by rank-and-file cultists, cult fanatics will leave the toe-to-toe fighting to them for the most part, instead using sacred flame and spiritual weapon to do damage from a distance (at the cost of only one 2nd-level spell slot—what a bargain!). They can use this combination over and over again, since sacred flame is a cantrip and spiritual weapon is a bonus action. However, before casting spiritual weapon, they’ll cast shield of faith, another spell that requires a bonus action, but only when it’s first cast. After that first turn, they can keep shield of faith going and also use their bonus actions to cast and direct spiritual weapon.
There are other tactical combinations cult fanatics can use on PCs who require special attention. For instance, they can cast command to order a troublesome opponent to “Approach!” then follow up on their next turn with inflict wounds. They can drop shield of faith to cast hold person and paralyze a PC, then rain critical hits on him or her with spiritual weapon (even better if that PC is already engaged in melee with one or more rank-and-file cultists) and enjoy automatic saving throw failures against sacred flame. And, as mentioned before, they can command a PC who’s engaged with multiple melee opponents to “Scram!” thereby granting every one of those opponents an opportunity attack.
In the episode “Recognized!” in chapter 4 of Hoard of the Dragon Queen, a PC is recognized by a cultist in the caravan, who attempts to assassinate him or her. In our group, the unlucky PC was the paladin, and for the sake of dramatic tension, I decided that the cultist who recognized him was in fact a cult fanatic. While the paladin slept, the cult fanatic cast hold person on him to paralyze him, then dragged him out of his tent, unarmed and unarmored, into the darkness beyond the caravan camp. At that point, the cult fanatic socked the paralyzed paladin with inflict wounds. I gamed this encounter out carefully beforehand, to make sure the paladin would be able to make his saving throw at the right time and survive by stacking abilities and spells such as Divine Smite and divine favor on top of unarmed melee attacks. As intended, he survived . . . barely.
Cult fanatics, as their name suggests, will fight to the death.
Acolytes are not really equipped for combat at all, except as backup. Their physical abilities are all average. They’re armed with clubs, representing improvised weapons (in a temple or shrine, a nice, heavy candlestick is a good go-to). Effectively, they’re commoners with above-average Wisdom and the ability to cast spells.
The key spells in their arsenal are bless, cure wounds, sanctuary and sacred flame. They’ll use bless if accompanied by allies who are engaging in real fighting, cure wounds on any ally who takes a moderate wound or worse, sanctuary on a key ally who requires protection or on any ally who’s seriously injured, and sacred flame in any other situation that would call for them to attack. But, like commoners, they’re as likely to freeze or flee as they are to fight; it depends entirely on the odds and on whom they’re with. If the PCs threaten their superiors, their worshipers or anyone under their protection, they’re more likely to fight than they would be otherwise.
Priests are also not well suited for combat and generally will fight only in self-defense, but they do have some tricks up their vestments. Notable among these is spirit guardians, a potent area-effect defense spell that requires concentration. If combat ensues, this is the first spell a priest will cast, forgoing his or her bonus action (which could otherwise have been used to cast spiritual weapon) in order to gain this powerful field of divine protection. On his or her second action, the priest will cast spiritual weapon as a bonus action and, if need be, the sacred flame cantrip, which does 2d8 damage rather than 1d8 because of the priest’s spellcaster level. If an enemy does close with the priest, most likely because the enemy’s Wisdom is high enough to resist the effects of spirit guardians, he or she will use Divine Eminence as a bonus action, then Attack (action) with his or her magically enhanced weapon. In most cases, however, the priest is looking to drive attackers away rather than kill them. A seriously wounded priest (reduced to 10 hp or fewer) will surrender or flee—unless the PCs are of an alignment directly opposed to the priest’s or worship a deity who’s an enemy to the one the priest worships, in which case the priest may see a glorious death battling infidels as his or her ticket to sweet treatment in the afterlife.
In the presence of allies, the priest has a couple of other options: After casting spiritual weapon, a priest may choose to cast guiding bolt instead of sacred flame at an enemy whom one of his or her allies is engaging: a hit gives the priest 4d6 radiant damage and his or her ally advantage on the next attack roll against that enemy. Also, like the acolyte, a priest can cast cure wounds on an ally who’s taken a moderate wound or worse, or sanctuary on an ally who needs an emergency bailout. (Remember that leveled spells cast as bonus actions, such as sanctuary and spiritual weapon, may be paired in the same turn only with cantrips, such as sacred flame—not with other leveled spells that are cast as actions, such as guiding bolt.)
Finally, druids are, interestingly, scrappier than your average clergyperson, with slightly above-average Dexterity and Constitution and the delightful shillelagh cantrip, which boosts their attack modifiers with staves from +2 to +4 and their damage from 1d6 to 1d8 + 2 (the MM says just 1d8, but this is a documented error) and costs only a bonus action. The big gun in their arcane arsenal, though, is entangle, an area-effect spell that has the potential to restrain enemies, granting them disadvantage on attacks and advantage on attacks against them—plus making them sitting ducks for damaging spells that require Dex saves. (Unfortunately, druids don’t pack any of those.)
Druids keep to themselves, so any encounter with one will be on his or her own home turf, most likely, and their goal in the event of a conflict will be to make the PCs go away (if they’re weaker) or to get away from the PCs (if they’re stronger). We’re looking primarily at two tactical combinations here. One is entangle (action) plus produce flame (action): first, restrain the PCs; second, while they’re restrained, lob flames at them with advantage for 1d8 fire damage a pop. The other is shillelagh (bonus action) plus a melee weapon attack (action), with thunderwave as a backup for when the druid is surrounded by three or more melee opponents and needs to take the heat off. Either may be effective for standing one’s ground, depending on circumstances, terrain and the opposition.
If a hasty departure is required, the druid casts longstrider (action), increasing his or her speed to 40 feet per round; then, on the next round, Dashing (action) off into the wilderness. (Barkskin isn’t worth the time it takes to cast; if the druid casts this spell at all, it will be before combat begins, and the druid will be down one 2nd-level spell slot.)