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What happens next is also unclear. The easiest thing to do is to assume targetting part of a swarm is the same as targetting 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 targetting 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.

The only definition of creature we have in 5e is from this tweet:

Swarm is not there. Beasts are. So a Swarm is indeed a collection of Tiny Beasts. A swarm is not a creature, it is a monster composed of creatures.

A DM could reasonably rule that treating a swarm as a creature is correct via rule 0, but RAW swarms are composed of creatures.

The effects of a creature within a swarm being targetted are not clear by the rules. So the effect could be marginal and/or determined by the DM. Or you could treat targetting a creature as part of a swarm similar to targetting part of a creature's body.

You could make a ruling that targetting a creature that is part of a swarm simply involves engaging with the entire swarm's game statistics.

The same kind of call has to be made for every spell or effect that targets a creature; is it reasonable that when targetting 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.

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:

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.

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.

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.

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.

Fireball text:

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.

A swarm is not a creature.

The only definition of creature we have in 5e is from this tweet:

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), or it could just hit one beast and make it drop dead (and thus do trivial damage).

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 targetting part of a swarm is the same as targetting 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 targetting 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.

The only definition of creature we have in 5e is from this tweet:

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).

This list is titled "types", and is:

Swarm is not there. Beasts are. So a Swarm is indeed a collection of Tiny Beasts.

"The variety of beings that count as creatures" in plain English means that the list is complete.

This list is titled "types", and is:

Swarm is not there. Beasts are. So a Swarm is indeed a collection of Tiny Beasts. A swarm is not a creature, it is a monster composed of creatures.

A DM could reasonably rule that treating a swarm as a creature is correct via rule 0, but RAW swarms are composed of creatures.