[RPG] If someone attacks a swarm with a single-target spell while it’s in a character’s space, is that character also affected by the spell


My players have walked into a Spider Lair and fought their first group of Spiderlings, or effectively a Swarm of Spiders from the MM. I didn't realize until after the encounter that they have a reach of 0 for their attack and that they can move through and stay on a space taken up by another creature (which occurred to me is the only way it can actually make its attack.) The party's going to fight more next session.

What happens when a swarm is in a character's space attacking them, and someone casts a single-target spell against that swarm such as Hellish Rebuke or Fire Bolt?
Since the swarm is invading that character's space (we assume because they are swarming around them or over them), is the character also potentially affected by the spell?

Best Answer

The only special rules we have for swarms are the following:

Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny [creature]. The swarm can’t regain hit points or gain temporary hit points.

So as far as the rules are concerned, swarms don't have any exception to the usual targeting rules, so there is nothing that would make spells hit the player. D&D 5e doesn't have any rules about firing into melee, or anything else which would cause someone to hit a different target to the one they aimed at.

However, since the swarm is swarming around the player, at least some of the swarm will have the player's body between them and any given spellcaster, providing the swarm with some degree of cover. A case can be made for different degrees of cover:

  • No cover - As long as you can see some of the swarm, you can hit them, and attacking a bit of the swarm is the same as attacking the whole swarm.
  • Half cover - About half of the swarm will have the player's body between them and the spellcaster, so the swarm as a whole has half cover.
  • Three-quarters cover - Unfortunately, I can't really see a case for this.
  • Total cover - Since some of the swarm obviously has total cover, you can't target the "whole" swarm. (This is pure sophistry, and I don't recommend trying it on your players.)

The rules for cover are on page 74 of the Player's Basic Rules; I won't write them all out here, but half cover gives +2 to AC and Dex saves, three-quarters cover gives +5 to AC and Dex saves, and you can't target anything that has total cover. How you rule is up to you, of course, but I'd probably go with half cover.

Of course, all of the above needs the usual disclaimer: You are the GM. You can houserule however you want, so if you think players should be at risk of hitting their allies, you're free to make that rule. I will say that you should be careful about doing this, because swarms can be very tricky for some groups to fight, particularly at low levels.