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I'm running the Curse of Strahd adventure for DnD 5e. The party is fighting Baba Lysaga, who already has 3/4 cover, giving her +5 AC as she flies on a skull. I was thinking about adding Swarms of Flies to occupy the same spot as the hag, as those are under her control. How would this affect the targeting of Baba Lysaga? Could a player pick her as a target when making an attack? An AoE would hit both, but what if the swarm works as an obstacle to line of sight?

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  • \$\begingroup\$ Welcome to the stack @Artmaton! Take the tour when you have a moment, and feel free to peruse the help center for more in-depth info about the site. \$\endgroup\$
    – Jack
    May 9 at 18:23

2 Answers 2

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Probably not

First, we can consider cover (PHB p.196). In general, a creature being between an attacker and the target grants half cover, which gives the target a +2 to AC. However, this requires "an obstacle that blocks at least half of its body", and since a swarm is made of lots of tiny individual bugs, there really isn't a solid object there to catch an arrow, and it doesn't seem reasonable to grant cover on that basis.

Second, the target could be obscured (PHB p.183). The lightly obscured effect can be caused by things like patchy fog or moderate foliage, so a swarm of flies probably fits into that category. Heavily obscured requires something like "darkness, opaque fog, or dense foliage" that "blocks vision entirely", so a bunch of flies probably doesn't fall into that category. Lightly obscured only applies a disadvantage on Perception checks that rely on sight; it does not hinder attacks in any way. (Heavily obscured basically causes the Blinded condition, which means attacks have disadvantage, but that doesn't seem reasonable to apply here.)

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  • \$\begingroup\$ If you can shoot an arrow deliberately at a swarm you can shoot one accidentally, so I don't agree that they wouldn't be solid cover \$\endgroup\$
    – SeriousBri
    May 9 at 17:41
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    \$\begingroup\$ I mean, game abstraction and all, but swarms are resistant to physical damage presumably because shooting or cutting the swarm means you're just killing a small number of the constituent creatures, so I would expect an arrow to go right through one even if we don't actually track that for game purposes \$\endgroup\$ May 9 at 18:09
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You can rule that way, but it won't help in this case

Creatures can provide cover to other creatures [PHB Chapter 9: Combat]:

A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.

And the DMG (p.251) adds, in the optional rules for playing on a grid:

To determine whether a target has cover against an attack or other effect on a grid, choose a corner of the attacker's space or the point of origin of an area of effect. Then trace imaginary lines from that corner to every corner of any one square the target occupies. If one or two of those lines are blocked by an obstacle (including another creature), the target has half cover. If three or four of those lines are blocked but the attack can still reach the target (such as when the target is behind an arrow slit), the target has three-quarters cover.

If the swarm was directly in between the character and the target, it would count as an obstacle blocking the target and thus provide cover. What is unclear is whether the target can be blocked by a creature in its same space. Since swarms are an exception to the rule that a creature cannot willingly end its turn in another creature's space, it is your call as a DM whether they are also exceptional in that they can block for a creature in their same space.

RAW, we assume that a creature controls the space(s) it is in, without regard to its size, height, substantiality, etc. A gelatinous cube [Ooze Cube. The cube takes up its entire space] blocks things on the other side of the space it is in just as much as an air elemental [Air Form. The elemental can enter a hostile creature's space and stop there], which blocks just as much as a giant fire beetle [size small and low profile]. If you as a DM want to permit the fly swarm to block the crone, you can - if you don't want it to, you can justify this by saying it can't block in the same space - you don't need to reference the size of individual flies or whether you can see through the swarm.

As Darth Pseudonym explains, if the swarm heavily obscured the crone, attacks on her would have disadvantage, but it doesn't seem reasonable for the fly swarm to provide heavy obscurement - and if it did, it would work both ways, and obscure her vision looking out as well. In any event, there aren't rules about when a creature obscures another creature, so that it your judgement call as a DM as well.

Thus, likely the only benefit the swarm provides is half cover, and she is already at 3/4 cover in her flying skull. Since cover doesn't stack, and you just apply the highest benefit, the swarm would not help her in the skull, although it could if she was forced to land and exit.

There is another optional rule you might consider - although it won't help her and could hurt the swarm. When someone is shooting at a target with cover (DMG 272):

When a ranged attack misses a target that has cover, you can use this optional rule to determine whether the cover was struck by the attack.

First, determine whether the attack roll would have hit the protected target without the cover. If the attack roll falls within a range low enough to miss the target but high enough to strike the target if there had been no cover, the object used for cover is struck. If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit.

The crone will be struck just as much as before, but the PC's will be able to whittle down the swarm surrounding her even when they miss their attacks on her. That might be fun for them, especially if she has left her skull and you have ruled that the swarm is providing her with cover.

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