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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?

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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.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – mxyzplk - SE stop being evil May 26 '18 at 14:15
  • \$\begingroup\$ Of note is the optional rule for Hitting Cover (DMG pg272), so if you rule that Swarms gain Half Cover from having another creature in their space (as I personally do), this means that most attacks that target a swarm will have a 10% chance to hit the creature in their space instead. I personally find this quite reasonable, provided you tell the players this upfront before resolving their attack, in case they choose not to risk it. \$\endgroup\$ – Taxi4Dave Jan 8 '19 at 18:35
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A swarm may enter another creatures space, but how big is that space?

A medium creature occupies one square on the grid, and one square on the 5th edition battle grid represents 5 feet. A creature in a square is not a solid 5X5X5 cube but a moving, dodging, threatening being. At the moment a spell, such as Fire Bolt is cast, for the sake of narrative, it does not have to be at the moment a swarm is necessarily crawling on said creature, but just within the same 5 foot space. Whoever is describing the results of the action (usually the DM) could describe it like this:

You point your finger and concentrate on the swarm. A bolt of fire launches from your fingertip and strikes the ground where the swarm of ants seems the most dense. Roll 1d10 fire damage against the swarm.

Or

The swarm of deadly mosquitoes temporarily disengages from your friend, and you take the opportunity to quickly shoot a bolt of fire into the mass of insects hovering in the air, inches from your ally. Roll 1d10 fire damage against the swarm.

Just because a swarm is within a creature's space doesn't mean you have to visualize (or narrate) that it is on the creature the whole time it is there. It is simply in proximity to the other creature, close enough to attack it.

An Alternative

For the more vindictive DMs out there, you could enforce disadvantage on the attack roll against the swarm, use the cover rules like suggested in the other answer, or even take inspiration from this part of the Fire Bolt spell:

A flammable object hit by this spell ignites if it isn't being worn or carried.

Certain spells may have dangerous environmental effects and, still using Fire Bolt as the example, a dry bush near the creature may catch fire and pose a danger or perhaps a piece of their clothing.

Of note, if you go this direction, is the Sculpt Spells ability of the Evocation Wizard found on pg.117 of the PHB. It allows the wizard to protect a limited amount of allies from dangerous spell effects that he/she casts. This would suggest, to me, that shooting a magical bolt of fire into your allies space should be a dangerous thing to do.

All of these options, though, are house rules and make the system more biased against the players than intended by the designers.

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