D&D 5E How to handle massive oncoming swarms

[Oofta]

I can't believe nobody has brought up Spirit Guardians and similar effects.

Before having any discussions like this, first make sure the party cleric (etc) doesn't have access to these spells.

Because if they do it's just game over: any monster that comes at you in great hordes must by definition be weak enough to die to the aura.

This kind of spell is simply created by a developer who hates the trope you're trying to enact here.

Then give the hordes ranged attacks and focus fire on the caster. Sooner or later the caster will fail their concentration check even if the ranged attacks are only doing minimal damage. You could also just treat it as an area effect spell. Yes it's effective but some still get through just based on shear numbers.

 

[Quickleaf]

Wow, love it! Could you share the entire stat block? How did attacks work? Could the horde attack more than one party member per turn?

Sure. The stat block is 4th edition and it's in my old conversion of Dragon Mountain (posted on these boards). That PDF doesn't read the stat block as text, but as an image, so I'm cutting-pasting the image, and attaching it here.

The intent was not simulationist and exacting. It was cinematic and abstract.

The horde made an attack against each PC in the "area." This occurred in a large multi-level dungeon with very specifically defined areas, and when a horde formed was determined by some moving dials & exploration skill challenge spanning the entire dungeon. My assumption was that the kobolds were canny enough to get about 4 kobolds able to attack each PC (in 5e parlance 2 kobolds Help the others, and average damage of the horde attack was ~2.5x a single kobold's damage), while the PCs were canny enough to use terrain and positioning to limit it so no more than about 4 kobolds could attack each of them.

The reason I'm not converting it to 5e here is that I'm not 100% convinced that a monster stat block is the right tool to get the job done. Since then, I've increasing come to think of a swarm as a hazard, and less as a monster.

It doesn't really handle the trickling stream of near unending monsters but this will take care of most situations other than a few edge cases. I'm really liking the trains and legendary actions.
I'm thinking I would have multiattack with the number of attacks dependent on the number of hit points. E.g. when the swarm is half the size it makes half the attacks.

In my opinion, the fatal flaw when approaching a swarm/horde encounter is focusing on the stat block and "traditional combat" aspect of the encounter. Yes, that's there, and it's good that you're hacking the system to get something that works in that department.

But the focus should be on inspiring creative lateral thinking. That's where the payoff is. For example, presenting a "near unending stream of monsters" is actually about the players discovering/creating a means by which to stop that stream. They might collapse a tunnel blocking the kobolds. They might use their sonic McGuffin to drive off the giant bats. They might charm the chuul and convince it to telepathically call off its hounds of tindalos.

I think the uninspired 5e swarm stat block – and how I've seen 5e handle swarms in official adventures – is a product of this flaw. In my opinion, swarms have a lot more in common with hazards/trap than with monsters, and the more we can incorporate such things (i.e. triggers, ways to avoid, escalation points, dynamic threats, countermeasures) into our presentation of swarms, the better our design will be & the better the play experience.

One last thing I'm thinking of is a trait that says that depending on the number of hit points left on the stat block, the horde auto hits everyone in the group up to a certain AC. Or that the horde starts with an insanely high to-hit bonus that is whittled down as hit points decrease. That more than anything would make hordes truly terrifying. Even characters of great power should fear getting caught up in a mob or horde.

The problem with auto-damage is the narrative of the horde, and mechanics you're setting up around it, are working against you.

You describe a balor with wreaths of flame tracing about its shimmering form, and players aren't going to bat an eye when they take damage from its Fire Aura.

However, have a bunch of kobolds, orcs, goblins, norkers, or whatever lay into them without touching the dice to roll and watch their reaction. When I did this with my group (when using the Handling Mobs rules in the DMG), there was stiff dissonance and objection. And rightly so. They have things on their sheet like Parry and shield just for occasions where they get melee attacked. Auto-damage from a horde is saying: "Yes, this is a monster. Look it has stats. Yes, it's attacking you. But no, that defense won't work against it. Because it's...uh...a special monster."

Another way to phrase this is that if you're going to make assumptions about monster effectiveness when functioning in a horde, then you'd better damn well make some assumptions in favor of your players too. Better yet, let them have the chance to use those cool abilities and save themselves.

The other problem is that your scenario – anyone below this AC gets hit – also doesn't map to how we imagine certain swarms. A swarm of fire ants sure as heck is going to find a way under your plate armor. But having that plate armor with visor is going to protect you from giant bats – at least from their damaging bites, not from being blinded and confused by their fluttering blocking your already limited sightlines of your visor.

My long-winded way of saying: Not all swarms are created equal. Design specifically.

A swarm is different from a horde. A swarm of fire ants functions differently from a swarm of bats. A horde of kobolds should threaten different things than a horde of orcs.

The only downside is that unless you are dealing with tiny or small creatures, I don't know if my players would accept "from all sides" if they are in a confined area. It is still difficult to determine how this will work when the party holds a choke point for example. This is where "The Horde Presses Down" can really come into play. I would further add that the horde can expend a certain number of hit points to give a bonus to the hordes contested skill check to represent sacrificing the lives of those in the front to make an overwhelming push.

Thanks again, this has given me some ideas that I'll definitely will be putting to use.

Yep, it's cinematic and abstract. I was also working within the context of a kobold dungeon littered with Tiny side passages and tunnels and vertical escarpments that kobolds could squeeze through. Breaking away from the confines of 2D thinking may help your group a little here (or maybe you already have, in which case disregard this point).

Just like any other combat, there's some common sense that you need to apply. If the players hunker down in a choke point that should affect how many orcs can reach them. Same same here, but I'd also argue that one of the countermeasures to facing a horde might be: Reach the choke point at Scarl's Gulch 240 feet away before the orc horde kills you. And reaching the choke point would cue an end to the fight against the horde, possibly a few independent orcs to deal with, but the main threat has passed (for now).

 

[MNblockhead]

I can't believe nobody has brought up Spirit Guardians and similar effects.

Before having any discussions like this, first make sure the party cleric (etc) doesn't have access to these spells.

Because if they do it's just game over: any monster that comes at you in great hordes must by definition be weak enough to die to the aura.

This kind of spell is simply created by a developer who hates the trope you're trying to enact here.

I did, in my OP. With the killer frog example, the party clearic set up spirit guardians and then the party used a cube of force to protect themselves and block the exit to basically make a frog blender.

With the idea of HP per square, I think the damage from spirit guardians, black tentacles, spike growth, etc. are easy to adjudicate quickly without all the bookkeeping you'd have to deal with if running a large number of individual NPCs. But the movement restrictions and saves are somewhat more difficult.

 

[CapnZapp]

Then give the hordes ranged attacks and focus fire on the caster. Sooner or later the caster will fail their concentration check even if the ranged attacks are only doing minimal damage.

No, there's nothing wrong with the horde or the DM. The fault lies with the spell.

In other words, this is just the "if Detect Alignment gives you problems, just give the bad guy a Hat of Non-Alignment" solution all over again.

The proper solution is to not allow spells like Spirit Guardians in a game where hordes of zombies etc is meant to be meaningful. The simplest solution (albeit not a very effective one) is to wish D&D designers get better at not short-circuiting common fantasy tropes. A pretty simple solution is to ban these spells (but still, doing what you paid WotC to do...)

The more involved solution is to maximize the damage a spell like Spirit Guardians can cause. Something along

after the spell has killed 2 creatures per spell level the guardians get momentarily overwhelmed and the aura shuts down until the start of your next turn

or

The spell can cause a maximum of 60 damage per spell level. After the aura has caused this much damage, the spell ends.

or similar.

Or simply acknowledge the concept of the spell simply is too powerful for certain tropes, and make it uncommon (to use Pathfinder parlance) so players can only access it with the explicit buy-in of the Dungeon Master.

 

[NotAYakk]

Nice! Another great idea I'm going to use.

A mixture of horde stat blocks and this HP/square idea will really help. I like how saves and morale works.

One thing I need to think about, which I might change is the to-hit bonus. If someone is surrounded by Zombies...not just 8, but a horde pressing in, even a +11 means you need to roll over 10 to hit most well armored fighters. Not sure how I want to tweak it. Maybe if you are are surrounded completely, they auto hit. If you are flanked the hit is at advantage.

So when "overwealmed" by a mob, the mob gets advantage and +1 to hit for every extra foe.

A tweak to overwealmed:

Overwealm: When hit by a melee attack by a mob, the mob can attempt to overwealm. This provokes an opportunity attack; the mob gets a bonus to its AC equal to its bonus on its attack roll (+1 per extra attacker, max +10). If the opportunity attack misses, the target is overwealmed; they are grabbed and restrained (DC 8+strength+proficiency+1 for every mob square adjacent to escape) and the mob moves into the square of the overwealmed creature.

Mobs can freely extend through squares of overwealmed creatures.

A skilled fighter holding a line or a choke point can prevent overwealmed ... until they get a bad roll. If surrounded it becomes much harder.

I like tying it into a reaction attack of "no, you aren't allowed, die". This makes fighter-types able to resist being overwealmed much better than non-fighter types, and boosts the damage melee characters get from being in contact with the mob.

The worst for melee foes is +8 and advantage (9 foes, including your square). Which in 5e bounded accuracy should be usually enough, and a character with crazy AC can avoid being hit (20 dex level 15 sword bard with +2 studded leather and a shield spell has 27.5 AC; with a +2 shield and haste has 33.5, which is roughly what you need to not be hit by an overwealming horde of under-CR-1 foes more than 10% of the time (!)). Damage is 4[W]+stat.

Also, if you have the party surrounded with the mob around them and between them do you roll for the mob to hit more than one player per turn? I'm thinking I would not give the mob initiative. Instead I roll to hit for the mob at the end of each players turn. Give the character an opportunity to move to a better location or decrease the number of attackers (hp per square). E.g. the mob auto-hits or rolls to hit after each characters turn if that character is adjacent to at least one mob square.

The horde's squares attack on the horde's turn. You work out which squares are attacking which players.

When in doubt the goal is to make it feel like normal monsters. Shortcuts (scaling damage, bonus to hit, save DC boosts, etc) are simulationist in nature, mathematical shortcuts to decently approximate what would happen if I actually rolled for every monster and accounted for cover etc.

Like, vulnerability to fireball is one thing; but by counting squares, the answer to "my fireball auto-kills zombies, why didn't it blast a 20' radius hole in them" gets answered. They get a huge save bonus (that scales with how many you try to take out) so take half damage, and it does blast a hole. The ones further from the center are only standing because they got cover from the raw amount of cooked meat between them and the center of the blast.

But hypnotic pattern and "mass save or suck":

Saving Throws: The Mob gets a +1 bonus to saving throws for every addition target of an AOE spell beyond the first square. If the creatures have evasion or similar, it converts to advantage on the saving throw. For a spell that doesn't have half effect on a successful save, use the margin of victory to work out roughly how many squares are taken out by it.

Ie, a "save or suck" spell hits 20 squares. They get a +19 bonus to their saving throw, and beat it by 12. This means 12 squares save, and 8 squares fail. The spell was faerie fire; so until 8 squares are killed, attacks on the mob are at advantage.

 

[MNblockhead]

This is definitely a thread I'm bookmarking to use to build my own home-brew rules on swarms and hordes.

@CapnZapp While I like the ability for a cleric to call forth spirit guardians, I agree that they should not be an infi-blender. I like the Guardian of Faith approach where after X amound of damage dealth, the spell goes way. I'd like an upper limit of damage output per turn or number of creatures it can affect per turn. You should be able to overwhelm spirit guardians. I think 5e's answer to this is the swarm stat block. You pool the HP of the swarm. The spirit guardians cannot kill everything in the swarm in one turn.

The HP per square helps with this as well.

 

[Puddles]

There's lots of great ideas in the thread. I haven't ran any swarms yet, but thinking about it now, I would want to focus on "what makes this cool for the players" rather than on how to make the swarm itself super detailed.

I love the idea of the players having to "carve a path" through a never ending swarm, so what I would probably do is change up the way rounds work so instead of each player moving on their turn, they instead move en masse at the end of each round. The distance they can move is based on how much damage they did together, (so there would be thresholds of damage to reach to unlock further distances). The swarm doesn't have a fixed position and is just presumed to be "on all sides", but movement might slow to a trickle, 5ft or so, for the adventurers if they are not killing enough.

Add a doom-clock into the situation to heighten the drama and perhaps have other actions that allow them to move further too (like waving a torch around, if the enemies are scared of fire).

For me, this would be cool for the players as they would get the feeling of wading through the horde as the sand runs down in the timer.

 

[Oofta]

There's lots of great ideas in the thread. I haven't ran any swarms yet, but thinking about it now, I would want to focus on "what makes this cool for the players" rather than on how to make the swarm itself super detailed.

I love the idea of the players having to "carve a path" through a never ending swarm, so what I would probably do is change up the way rounds work so instead of each player moving on their turn, they instead move en masse at the end of each round. The distance they can move is based on how much damage they did together, (so there would be thresholds of damage to reach to unlock further distances). The swarm doesn't have a fixed position and is just presumed to be "on all sides", but movement might slow to a trickle, 5ft or so, for the adventurers if they are not killing enough.

Add a doom-clock into the situation to heighten the drama and perhaps have other actions that allow them to move further too (like waving a torch around, if the enemies are scared of fire).

For me, this would be cool for the players as they would get the feeling of wading through the horde as the sand runs down in the timer.

I agree, this is something where group movement would be a good addition. Or just add delay back in from previous editions as people hesitate momentarily waiting for the slowest to react person.

Can also be handled with mobs by just having the mob(s) go on the same initiative count.

 

[MarkB]

If the idea is just to get in and get out from a certain point while effectively endless hordes are swarming you, I'd probably structure it more as a chase or skill challenge, rather than a combat. Basically treat the horde as a debilitating environmental effect to be overcome.

 

[Stalker0]

I can't believe nobody has brought up Spirit Guardians and similar effects.

Because the thread was about creating swarm stats, not how to beat swarm stats.