How can I combine numbers of low CR creatures so that using them doesn't bog the combat down?

Question

I have a whole series of encounters built around various kinds of spiders, controlled by some significant baddies in service to Lolth.

At the bottom of this arachnid hierarchy is a large number of chitines. Thematically they fit well within the scenario, but I was not satisfied with my first session using them. Many of them were initially killed with areas of effect attacks, which was great, but then I tried having them move and attack in groups, and I winged the mechanics and math of it. It went okay, but I'd rather have a little more structure than just wing it.

What I would like, maybe, is a way to take the stats of x number of critters (chitines in this particular case) and combine them into a group, so that the stats of the x critters are reflected in the group.

I read through the section on mob combat in the DMG, and I think that is the sort of direction to go in, to combine numbers of critters into a group, but the DMG calculations seem complex to handle on the fly, and I don't want to have to consult a table. It would be great to have a formula or method to translate x critters in a group into y to hit and z damage, and so on.

So, my question is: How can I combine numbers of low CR creatures so that using them doesn't bog the combat down?

Also, I've read this question and I don't think it's a duplicate of mine, since I'm specifically talking about low CR creatures.

Answer 1

Roll once.

You can employ a "roll once" tactic that basically allows you to roll once to hit, and damage. This is very generalised, and obviously doesn't allow for different AC's, etc.

Normally, you would only roll one Initiative roll for each type of enemy, so this would mean that they all go at the same time. This is fairly standard so as to not bog down the initiative order, trying to keep track for which one "Chitines G" was. In similar fashion, you can also do this for attack and damage rolls.

Basically, on their turn, just roll a d20, add any modifiers, and then if that hits anyone, you can deal damage. The only thing you will need to consider is dealing extra/half damage for resistance rolls, which could probably be dealt with on a case-by-case basis easily enough.

Add a "gang up" bonus

Some systems (like Pathfinder) allow for a flanking bonus, or something similar, that allows one creature to aid another to grant a +2 bonus to hit and damage. In this situation, positioning is important - only creatures that are one direct opposite sides give this bonus, but that can be up to you. Additionally, if you feel it can be evenly balanced, additional creatures can forfeit their attacks to further boost this bonus.

Swarms

Swarms can be similar to the gang up bonus - the main difference is the reliance on the overall health of the swarm. If they are above half heath, they deal full damage, and if they are below half, they deal half damage.

Creating a swarm does increase the overall CR of the monster(s), for example, rats are CR 0, and a swarm of rats is CR 1/4. The main differences are the damage, and HP that the monsters have, as well as an apparent "mob mentality" (i.e. Int and/or Wis take a bit of a hit), so you can potentially create a balance for your own "swarm" if you like too.

Answer 2

What I Ended Up Doing

I ended up using a combination of pencil and paper and one of the features of Roll20, which we use whether we're geographically distributed or co-located.

Rolling Attacks

In Roll20 it turns out there's a formula for rolling n 20-sided dice, adding t to each die, and counting the number of rolls that exceed a certain threshold.

If you want to roll 10 to-hits, each with a +5, and find out how many hit AC 15, in Roll20 you would use the command:

/roll {10d20+5}>15

Roll20 will then come back with a result, for instance, "5 successes".

Rolling Damage

I could not make the Roll20 interface roll n damage dice and add 1 to each. It would only roll n damage dice and add 1 to the total. Since the critters do 1d4+1 per hit, I considered using:

/roll {nd4}+n

for example:

/roll {5d4}+5

but in the end I ended up using the average damage from the critters' monster entry. I could just do the math in my head to find out the damage.

Combining Critters

I decided the critters would attack in groups of 4. I was intending on groups of 4 to 8, but I actually only did groups of 4.

When a group would attack a PC I would use a single icon to represent the group in Roll20.

I would write down on my notepad, SN-A, SN-B, etc, for each group of critters. (SN was my personal notation for small nasties, which is what their icon said.) Then I would write down four 18's below the heading for that group. (Each critter started out with 18 HP.)

Making Attacks

Each critter makes 3 +4 attacks. I was able to quickly know, okay, that's 12 attacks against AC 15, and I could enter in Roll20:

/roll {12d20+4}>15

It would tell me the number of successes, and I would just calculate the damage in my head. In the case above, if Roll20 told me "4 Successes", I would just do the multiplication in my head and say, 20 points. I would actually narrate it a bit, describing them trying to climb up the PC, or duck under a shield, etc., but in the end that would be the damage.)

When PC's would attack the critters, I was pretty liberal in dishing out the damage. Often if a critter took a near-fatal hit, I would cross that one off and narrate that it ran away. When a group dropped to 2, I would have them run away.

I ended up with as many as 3 groups of the critters with a handful of other monsters, all going on their own initiatives.

In Conclusion

I was pretty happy with the result. I could roll the attacks and manage the critters fast enough that it didn't feel like it was bogging down combat at all.

Using Roll20 was pretty important to the solution. I suspect there are other die rollers out there that I might like better, but Roll20 was great because my players could see my rolls without having to add another app to what we use or even switch to a different one.

If I were to do this in a group where we weren't using Roll20 I would look for a different die roller to allow me to roll large groups of attacks.

One concern is that the damage was a bit swingy. Some groups got wiped out before they ever dealt a point, and one group nearly single-handedly took out the group's 5-level fighter tank. But in the end, I liked that they were easy to kill, but dangerous enough in groups to need to be taken seriously.