What's the largest creature a swarm of stirges could kill in one round?

Question

I have been working on the challenge of designing encounters that must be passed with other talents besides combat.

One of those potential encounters is an area like a cave filled with hundreds of tiny, flying creatures like stirges.

To low level characters, such a swarm could require new solutions other than a single combat. A large number of these creatures could potentially attack in one round by swarming and covering every part of an enemy's body.

It would be great to show the power and danger of this swarm by showing a big creature getting attacked by the swarm, drained and killed in just a round.

It would be nice if that scene fit the rules of the game.

Tiny creatures are described as taking up 2.5'x2.5' on a battlemap. Four tiny creatures can fit into a single battlemap square. But this doesn't address the opportunity of three dimensions made possible by flying. Additionally, some large or very large creatures have fewer hit points than others and so could more easily be taken down by the swarm of stirges.

Perhaps there is even a lower hit point huge or gargantuan creature that could be taken down.

What is the largest creature from the game that stirges could realistically kill (i.e. 50% chance or higher) in one round using the standard combat rules of the game and existing published creatures? (The bigger the size and higher the hit points the better.) (We don't use flanking rules but it is fine to use them in a calculation if you do.)

Answer 1

The stirges could down a gargantuan creature

The average damage a single stirge does follows the pattern of the following table:

\begin{array}{c|c|c|c} \text{AC 10} & \text{AC 12} & \text{AC 14} & \text{AC 16} & \text{AC 18} & \text{AC 20} & \text{AC 22} & \text{AC 25+}\\\hline 4.525 & 3.975 & 3.425 & 2.875 & 2.325 & 1.775 & 1.225 & 0.4 \end{array}

Then we can look at the surface area of creature spaces based on the following table (in terms of number of tiny flying attackers that can hit the target)¹.

Note that some attackers can attack from more the other side of a different strige since their reach is 5 feet but they only take up a 2.5 sq feet space. When doing so, the target is considered to have +2 AC from the cover rules (I labeled these as "x more" in the table):

\begin{array}{c|c|c|c} \text{Tiny} & \text{Small/Medium} & \text{Large} & \text{Huge} & \text{Gargantuan}\\\hline \text{8 & 16 more} & \text{12 & 20 more} & \text{20 & 28 more} & \text{28 & 36 more} & \text{36 & 44 more} \end{array}

¹ I chose to work in 2-dimensions since the creatures space rules on a grid only incorporate squares.

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With that information, we can look at gargantuan creatures that have low AC and low HP. The best example of this is the brontosaurus from Volo's Guide to Monsters which has:

Armor Class 15 (Natural Armor)
Hit Points 121 (9d20 + 27)

• With an Armor Class of 15, a single Blood Drain attack deals an expected 3.15 damage.
• When the target has cover, the AC is 17 meaning a single Blood Drain attack deals an expected 2.6 damage

Against the brontosaurus, we can have 36 attackers without cover, and up to 44 more with cover.

With all 80 stirges, this deals an average of 227.8 damage per round, which is enough to kill the brontosaurus in one round. The minimum number of striges to down a brontosaurus in this way is 39 (36 without cover and 3 more through cover).

Answer 2

Probably just about anything you want

Bounded accuracy makes this a lot more feasible than you might expect. Unlike 3.5 where a 20th level character could utterly destroy low-level nothings with general impunity, that's not the case with 5e.

First of all, let's do some math to figure out how many stirges could actually attack this creature. I'm making the following assumptions:

• X = Square root of the number of squares occupied by the creature on a 2-d plane. So X=1 for a medium creature and X=3 for a huge creature.
• From your question, I'm assuming 4 stirges can occupy a 5'x5'x5' cube for battle purposes.
• Any creature being considered is assumed to occupy a cube. Thus, a medium creature occupies a 5'x5'x5' cube.

From these assumptions, we generate the following equation to determine how many stirges can attack a creature:

• Number of squares around creature = [(X+2)*2 + 2X] * X
• Number of squares above creature = (X+2)^2
• Total is multiple by 4

From this we find the following:

Medium (X=1)

• Number of squares around creature = 8
• Number of squares above creature = 9
• 17 squares * 4 = 68 stirges

Large (X=2)

• Number of squares around creature = 24
• Number of squares above creature = 16
• 40 squares * 4 = 160 stirges

Huge (X=3)

• Number of squares around creature = 48
• Number of squares above creature = 25
• 73 squares * 4 = 292 stirges

Gargantuan (X=4)

• Number of squares around creature = 80
• Number of squares above creature = 36
• 116 squares * 4 = 464 stirges

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So what can you kill with 464 stirges? Depends on how often they'll hit, but with a +5 to hit coupled with bounded accuracy, you're looking at quite a few hits.

I did a skim of gargantuan creatures and to exemplify my point, I picked:

Ancient Blue Dragon

With an AC of 22 and 481 hit points, this is a CR23 monster that notably does not have any resistance to non-magical attacks.

The stirges can only hit with a 17 or better. So on a roll of 17, 18, or 19 they'll deal an average of 5.5 damage. On a roll of 20, they'll deal an average of 8 damage. Thus, every 20 stirges that attack will deal, on average, 24.5 damage; this means the average damage per stirge is 1.225 damage.

With 464 stirges attacking the Ancient Blue Dragon, in one round they will deal approximately 568.4 damage.

At 18% more than the dragon's max hit points, you could reasonably say that dragon is definitely dead.

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Maybe we can do better with something that has resistances, so lets try:

Astral Dreadnought

AC of 20, 297 hit points, resistance to non-magical attacks.

The stirges hit on a 15 or better, but damage dealt is halved. On a roll of 15, 16, 17, 18, or 19 they'll deal an average of 2.25 damage. On a roll of 20, this will improve to 4 damage. So for every 20 stirges that attack, you're looking at 15.25 damage and average per stirge damage of 0.7625.

This leads to a 1 round damage output of 353.8 damage, about 20% more than the max. So this is also reliably killed.

Conclusion

If you want a gargantuan creature dead, and it's not immune to normal weapons, 464 stirges should do the trick.

Answer 3

Ok this is going to be a bit difficult given how there are no solid rules in such a situation, but we can make a few assumptions.

• We know stirges are tiny, on a battle map as per regular rules 8 of them could attack a small/medium creature in a single round (Blood Drain does have a 5-foot reach).
• They do 5 damage on average each attack thus could deal about 40 damage on average (provided all attacks actually hit) in one round.

Note that we are assuming that just one layer of stirges attack, logically you could be attacked by more but rules for 3d combat are not that fleshed out yet. (Given the height of most medium humanoids you could say they were attacked by 16 in a single round for 80 damage since the stirges are small enough and medium humanoids are tall enough)

In the case of a large creature on the basic grid, it can be surrounded by 16 stirges for an average of 80 damage (again assuming all attacks hit), but again thats assuming only one 'layer' of stirges actually attack. Depending on how tall your large humanoid is you could have 3-4 layers of stirges attack the beast at once. This is a bit overkill as you would be doing over 200 damage in this scenario if you attack a helpless target.

Following the rules to the letter, a batch of 16 stirges could take down a Winter Wolf (13 AC & 75HP) in once round( with over a 50% chance provided they had advantage on their attacks - i.e the wolf was prone). If you are willing to house rule 'layers' in combat and were willing to use 48-64 stirges you could very easily start taking down young dragons (also large creatures with 17-18 AC and 110-178HP) in the same situation. (imo - this is a bit unrealistic)