D&D 5E Urgent, for tonight's game: Making a Swarm

[jayoungr]

Does anyone know of a good guide to creating swarms out of low-level monsters?

 

[robus]

These days I would use GiffyGlyph’s Monster Maker and just pick the challenge level I want, plug in the stats and then describe the monster as a swarm of whatever.

 

[robus]

Especially if it’s urgent!

 

[prabe]

The Midgard books have a few monters as swarms and as individuals, IIRC, and the Monster Manual Expended has a swarm of stirges that might serve as a basis of comparison if nothing else.

 

[GMMichael]

For 5e, just a few easy steps:

-Start with your base creature.

-Expand its size based on number of creatures and their required combat spacing. Other characters can enter this space.

-Multiply hit points by the number of swarm members.

-The swarm gets advantage (on pretty much everything) if its target is within its space. Disadvantage on anything without.

-Every PC in the swarm fights against the stats of the base creature, but does damage to the collective hit point pool.

-At 50% hit points, the swarm disperses. Stragglers (maybe the leaders/big guys) remain.

I think that covers everything...

 

[jgsugden]

Quick and dirty modifications that work for most low level beasties and humanoids.

• Start with a single version of the creature.
• Increase the size category by 2 (tiny to medium, small to large, medium to huge).
• Double the hit dice and then add 5 hit dice.
• Add 6 to the strength. This may impact attack bonuses and damage.
• Give it bludgeoning, piercing and slashing resistance.
• Give it charm, fright, grap, paralyze, petrify, prone, restrained, and stun immunity
• Give it the swarm ability.
• Add 2d6 to the damage of the creature. If the creature is now large or bigger, give it (or modify) a multiattack action allowing 1 extra attacks.
• If it can cast spells or uses an ability that operates like a spell, give it 2 swarm actions (which are essentially legendary actions) that allow it to use the ability or spells at the end of another creature's turn. Quadruple any available uses or spell slots.
• Generally, increase CR by 3 levels (1/4 -> 1/2 -> 1 -> 2, 1 -> 2 -> 3 -> 4). Compare to existing monsters to fine tune.

Swarm of Bandits
Huge swarm of medium humanoid (any race), any non-lawful alignment

Armor Class 12 (Leather Armor)
Hit Points 49 (9d8 + 9)
Speed 30 ft.

STR
17 (+3)
DEX
12 (+1)
CON
12 (+1)
INT
10 (+0)
WIS
10 (+0)
CHA
10 (+0)

Senses Passive Perception 10
Languages Any one language (usually Common)
Challenge 1 (100 XP)

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a medium humanoid. The swarm can't regain hit points or gain temporary hit points.

Actions
Multiattack. The bandit swarm may make 2 attacks with any combination of scimitar or light crossbow attacks.
Scimitar. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 13 (3d6 + 3) slashing damage.
Light Crossbow. Ranged Weapon Attack: +3 to hit, range 80 ft./320 ft., one target. Hit: 12 (1d8 + 2d6 + 1) piercing damage.

Swarm of Drow
Huge swarm of medium humanoid (elf), neutral evil

Armor Class 15 (Chain Shirt)
Hit Points 49 (11d8)
Speed 30 ft.

STR
16 (+3)
DEX
14 (+2)
CON
10 (+0)
INT
11 (+0)
WIS
11 (+0)
CHA
12 (+1)

Skills Perception +2, Stealth +4
Senses Darkvision 120 ft., Passive Perception 12
Languages Elvish, Undercommon
Challenge 1 (100)

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a medium humanoid. The swarm can't regain hit points or gain temporary hit points.

Fey Ancestry. The drow has advantage on saving throws against being charmed, and magic can’t put the drow to sleep.

Innate Spellcasting. The drow’s spellcasting ability is Charisma (spell save DC 11). It can innately cast the following spells, requiring no material components:
At will: dancing lights
4/day each: darkness, faerie fire

Sunlight Sensitivity. While in sunlight, the drow has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Actions

Multiattack. The drow may make 2 attacks with any combination of scimitar or hand crossbow attacks.
Shortsword. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 13 (3d6 + 3) piercing damage.
Hand Crossbow. Ranged Weapon Attack: +4 to hit, range 30/120 ft., one target. Hit: 5 (3d6 + 2) piercing damage, and the target must succeed on a DC 13 Constitution saving throw or be poisoned for 1 hour. If the saving throw fails by 5 or more, the target is also unconscious while poisoned in this way. The target wakes up if it takes damage or if another creature takes an action to shake it awake.

Swarm Actions
The swarm can take 2 swarm actions, choosing from the option(s) below. Only one swarm action option can be used at a time and only at the end of another creature's turn. The swarm regains spent warm actions at the start of its turn.

Spellcasting. The swarm may cast a spell.

 

[Oofta]

I like @jgsugden suggestion, another option is to use the mob rules from chapter 8 of the DMG. Basically take their damage and determine how many monsters will hit on any given turn.

The example they give: eight orcs surround a fighter. The orcs’ attack bonus is +5, and the fighter’s AC is 19. The orcs need a 14 or higher to hit the fighter. According to the table, for every three orcs that attack the fighter, one of them hits. There are enough orcs for two groups of three. The remaining two orcs fail to hit the fighter.

Mob Attacks

d20 Roll Needed	Attackers Needed for One to Hit
1–5	1
6–12	2
13–14	3
15–16	4
17–18	5
19	10
20	20

I use this especially when the bad guys have ranged attacks or a mix of range and melee.

 

[the Jester]

I'll be happy to take a crack at it if you tell me what the base creature is and what CR you're trying for. I have a long history of making creatures into swarms, from 3e to 5e.

 

[dave2008]

Does anyone know of a good guide to creating swarms out of low-level monsters?

I'm not sure what you looking for, but I have some swarms in my 5e Hardcore Monsters thread:

• Vargoulle Swarm (CR 21)
• Imp Swarm (CR 21)
• Lemure Horde (CR 9)

EDIT: It is pretty easy to take one of those and modify with a different monster. One thing that is different with my approach is to use Legendary actions to make them a bit more dynamic.

 

[jayoungr]

I'll be happy to take a crack at it if you tell me what the base creature is and what CR you're trying for. I have a long history of making creatures into swarms, from 3e to 5e.

Twig, vine, or needle blights; would like them to be about CR 5-6. Thanks!