So a player casts sacred flame on a swarm. What would the effect be?
Would it take no damage, 1 point of damage, 1/2 damage or full damage?
It appears that there are no general rules for swarms that are provided in the rules, but some information can be gleaned from looking at individual stat blocks.
Looking at Swarm of Ravens for example we can see:
The Swarm of Ravens is listed in one entry in the MM, with one statblock, one pool of HP, AC, abilities, etc. Nowhere does it say how many individual beasts compose the swarm, nor does it say that they are still considered individual creatures.
The swarm is always referred to in monolithic form
The swarm can occupy another creature's space and vice versa , and the swarm can move through any opening large enough for a Tiny raven . The swarm can't regain hit points or gain temporary hit points.
The swarm has damage resistances and condition immunities different from those of normal ravens, is listed with a size of Medium (Ravens are Tiny) and does considerably more damage than a group of individual ravens would. This means that this is treating this group of creatures as an entirely new creature.
All these things show that all the rules we have for swarms treats them as one single entity not as a collection of smaller creatures. Thus, it is highly reasonable to extend that to targeting as well.
A swarm has one pool of hitpoints. Even if you decided to rule that a swarm contains an undefined number of individual creatures it doesn't change the result here. Since the swarm has one pool of HP for the entire group, even if you do only target one of the ravens in the swarm with sacred flame, it still comes out of the group pool and thus does the exact same amount of damage as treating the swarm as one creature for targeting.
Swarms get resistance to bludgeoning, piercing and slashing damage by virtue of being a swarm. Sacred flame doesn't count as any of these; it is radiant damage. Unless the swarm has resistance to radiant damage as well (which as far as I know none of them do), the swarm takes damage just like a normal creature would from sacred flame.
Because the swarm has its own stat block, like an orc or a dragon or a manticore, it is treated mechanically like a creature when it comes to spell effects and attacks. It is worth noting what a swarm is and how it is created:
They form as a result of some sinister or unwholesome influence ... even Druids can't charm these swarms, and their aggressiveness is borderline unnatural. (MM p. 337)
You will note that a swarm of ravens has a series of immunities and resistances that a regular beast does not have.
Damage Resistances bludgeoning, piercing, slashing
Condition Immunities charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned (SRD p. 391, Swarm of Ravens)
That's another point in not treating it like a single raven multiplied X times. Ravens have no such immunities. (SRD p. 387, Raven)
In aggregate, the nature of "swarming" makes a bunch of single creatures act collectively and thus pose a greater danger to a party than a few rats, piranhas, ravens or bats. (Consider that a squad of soldiers or goblins is far more dangerous than a single soldier, but since there is no "swarm of goblins" stat block that analogy ends there).
A Swarm does more damage in "swarm" form by a significant amount: the swarm does 2d6 and a raven does 1. For the purposes of melee and movement (in formation, however chaotic) a Swarm is treated as a 'creature' even though the Swarm is made up of a bunch of smaller creatures.
Once you adopt the "keep it simple" approach, the answer to your question is very clear:
The swarm counts as a single creature for targeting and damaging it in combat. So sacred flame behaves exactly as it would for any other kind of monster: the swarm makes a Dexterity save; if it fails, it takes the full damage to its hit points listed in its stat block; otherwise, there is no effect. That is, if you already know how sacred flame works in general, there's nothing more to know.
A swarm of any particular kind is attacked as if it were a single target because it is listed as a monster with a stat block with no indication of any special exceptions about how to target or damage it due to its swarm nature.
In other words, if there were something special you needed to know to adjudicate effects targeting or damaging a swarm, the rules would tell you. They don't, so there's nothing special to know. This is a fundamental design concept in 5e: things do what they say and only what they say, and there are no hidden rules.
This is also the intent of Mike Mearls, the lead designer for the Monster Manual. This is his response to me asking him about it (includes minor reformatting for this post, original here):
If it's one stat block, it's one creature, though note that a swarm might have special abilities that interact with things like opportunity attacks.
Therefore, the rules for sacred flame or any other game feature make no distinction between a swarm kind of monster and any other kind of monster. So, you're not missing anything, you're just maybe overthinking it.
For sanity's sake, we have to note that the swarm is literally made of multiple creatures, but, for the sake of targeting effects, dealing damage, and otherwise interacting with it, it counts as a single creature in the game. There are also some contexts where the multiplicity of a swarm could be relevant (as Mearls implied above), but sacred flame is not one of those contexts.
One thing is quite clear:
A swarm is not a creature. So your spell, which targets a creature, targets one of the beasts in the swarm.
What happens next is also unclear. The easiest thing to do is to assume targeting part of a swarm is the same as targeting the whole swarm with an attack spell. The swarm makes a save, and on failure takes radiant damage. This, however, is never made explicitly clear in the rules of 5e.
It does not, in general, make sense to treat a swarm as a creature in all contexts. Treating targeting a part of the swarm as being able to damage the entire swarm results in a reasonable combat simulation 99 times out of 100; you just assume attacks "cleave" through one component of the swarm into others.
From sage advice:
Can conjure animals summon a swarm? No. Conjure animals summons individual creatures, and swarms are groups of creatures.
In addition, we have:
The introduction to the Monster Manual lists the variety of beings that count as creatures, organized by type
-- Jeremy Crawford (@JeremyECrawford)
"The variety of beings that count as creatures" in plain English means that the list is complete.
This list is titled "types", and is:
Aberrations, Beasts, Celestials, Constructs, Dragons, Elementals, Fey, Fiends, Giants, Humanoids, Monstrosities, Oozes, Plants and Undead
(with the description of each not included).
Swarm is not there. Swarm is explicitly stated as not a creature in Sage advice.
A swarm is not a creature, it is a monster composed of creatures.
Spells that target creatures cannot target the swarm as a whole without DM's permission. It must target a single creature within the swarm (which in this case are Beasts).
The effects of a creature within a swarm being targeted are not clear by the rules. So the effect could be marginal and/or determined by the DM. Or you could treat targeting a creature as part of a swarm similar to targeting part of a creature's body.
If you attack a creature's tentacles in D&D, barring a DM's ruling this simply attacks the creature's AC and deals damage to its HP. In this case, we have a monster that is not a creature, but is composed of creatures.
You could make a ruling that targeting a creature that is part of a swarm simply involves engaging with the entire swarm's game statistics.
This, however, is a ruling. You should not, in general, treat the entire swarm as a creature.
As an example, Wild Shape which lets you transform into a Beast (a kind of Creature) does not let you transform into a Swarm according to this tweet:
Wild Shape lets you transform into a single beast. A swarm is a collection of beasts, not one.
-- Jeremy Crawford (@JeremyECrawford)
The ruling, which seems to be intended, is that attacks in general "blow through" individual creatures and kill larger numbers of them, damaging the swarm as a whole. This both explains why a swarm cannot regain HP (as damage to it involves killing creatures, which mere healing cannot recover), and why it has resistance to certain damage types (which, presumably, don't "blow through" as well).
In this particular case we have a radiant spear of light. It would be reasonable to both have this spear blow through multiple creatures (and thus do non-trivial damage), or it could disrupt the dark magics that unify the swarm (and thus do non-trivial damage),
they form as a result of some sinister or unwholesome influence
-- MM pg 337
or it could just hit one beast and make it drop dead (and thus do trivial damage).
The same kind of call has to be made for every spell or effect that targets a creature; is it reasonable that when targeting an individual creature in a swarm that the effect "blows through" to the rest of the swarm or not.
Similarly, ruling should be made when area effect damage is applied to a swarm. A large-volume fireball hitting a swarm of ravens that deals more than 20 damage per creature would reasonably instant-kill the entire swarm, even though the swarm has more than 20 HP left, as the fireball by the rules deals 20 damage to each creature (beast) in the swarm.
Each creature in a 20-foot radius Sphere centered on that point must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.
A Swarm is a collection of creatures, and Fireball targets each and every creature within its radius. By RAW, each creature in that swarm has to make a Dexterity saving throw, and take either full or half damage.
We could step back to a previous edition of the game, which would have been on the minds of the developers. In 4e D&D swarms were a kind of monster. They were vulnerable to area effect damage and took half damage from "single target" attacks. They were immune to forced movement from single target attacks, they can move through any opening large enough for the constituent beasts and they can occupy spaces of other creatures.
3 of these rules remain -- half damage from weapons (akin to the single target attack rule of 4e), squeezing rule, and overlapping spaces. The rest (that they are explicitly creatures, they take more damage from area attacks, and forced movement invulnerability) were stripped.
You could consider the stripping on purpose, or you can consider the stripping to be an example of simplifying rules and letting the DM make reasonable calls based off of the in-world fiction.
