The swarm takes radiant damage if it misses the Dexterity save
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)
- This points to a magical (something) being inherent in a swarm that keeps it together in this form. While the MM entry may raise the question of whether or not a Swarm is a single creature in your mind, the consistent thing to do is treat it as a single creature since it is statted with its own stat block.
- 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 should be 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:
- Roll the d20 against the spell DC for the caster
- Add +2 for the 14 dexterity score of the swarm,
- See if the swarm is damaged or not, based on missing or making the save.