Swarms and Followers
Summons and hirelings handled at an individual level can significantly complicate the ability of a dungeon master to run a game, both slowing down initiative and providing an undue amount of power creep. Often, swarms completely surpass all other strategies in both damage and versatility. The following rules are presented to control some of these somewhat problematic issues, while still allowing this otherwise enjoyable and flavorful playstyle to work in the context of an ordinary game.
Basics of Swarms
When you have multiple followers, they organize themselves into small groups called "swarms". A swarm of followers contains up to 10 statistically identical creatures, and a single player character may only have 3 swarms accompany them in initiative. Creatures organized in swarms are mostly treated as a single creature, acting, moving, and defending themselves in unison. Effects that target a single creature can also target a swarm of creatures, acting as normal, but due to their organization, swarms are much heartier and more dangerous than the individuals that compose it.
Swarms can be composed of any kind of creature - one might be a gang of thugs you have hired, a horde of zombies that you have raised, or possibly even single heroic NPC that has sworn themselves to you. Anything that is taking your orders is considered part of a swarm, but each statistically unique creature must use another of your 3 swarm slots if it accompanies you in initiative.
You might have more followers than can accompany you in initiative. Such followers are assumed to stay away from combat and defend themselves, and will not be subject to damage unless you call them into initiative after other followers fall or retreat.
Hit Points of Swarms
Instead of tracking each creature in the swarm's health independently, you should instead keep track of the total damage taken by the swarm. When the damage taken exceeds the health of a single individual, remove one unit from the size of that swarm, and reduce the total by the hit point maximum of a single individual in the swarm. If needed, repeat this process until the total damage taken is below the hit point maximum of a single unit in the swarm.
A swarm of creatures has the swarm creature property, may not gain hit points or temporary hit points. However, if two swarms are within the movement range of one another, you may transfer any number of individuals from one one swarm to another on your turn (no action required by you), increasing their effectiveness.
If you have more creatures of the same type as a given swarm following you, you can add those creatures outside of initiative. Alternatively, if you are standing the same space as a swarm, you may use a bonus action to add any number of individuals to the swarm, though the swarm will be stunned until the start of your next turn. This stun effect cannot be resisted.
When an attack or effect would target a single creature, it effects the swarm entire swarm as if it was a single creature, meaning that it may be able to damage more than 1 creature in the swarm if it deals enough damage.
If a damaging area of effect includes the entire swarm, the damage of that effect is applied a number of times equal to the number of creatures in the swarm. If an area of effect only reaches to the center of the swarm, the damage of that effect is applied a number of times equal to half the number of creatures in the swarm. If the area does not include the center of the swarm, the effect does not target the swarm.
If damage to the swarm would have cause some other effect, such as forcing a concentration saving throw, this save is made using the unmodified damage.
Size of Swarms
A swarm's size is increased in comparison to that of an individual - one size category larger than an individual if 3 or less individuals are in the swarm, or two categories larger otherwise. Swarms have the swarm creature trait, which means that the swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for any one individual without squeezing.
A swarm is always considered to be within 5 feet of an ally for the purposes of abilities like pack tactics or sneak attack.
Difficulty Classes and Rolls
When a swarm makes an attack roll, ability check, or saving throw, they can choose to add the number of creatures in the swarm to the roll instead of their ordinary modifier, as long as it uses their highest ability score or they are proficient in the given roll.
When calculating the DC for an ability of creatures, they may use either the ordinary value, or 8 + the number of creatures in the swarm. For example, a wolf's bite attack forces a DC 11 Strength saving throw. A swarm of 10 wolves may instead force a DC 18 Strength save.
This also applies to spell casters - a single cult fantatic has a spell save DC of 11, but a swarm of 5 cult fanatics has a spell save DC of 13.
Damage from Swarms
A swarm uses the normal attacks and damage that an individual would have access to. For each creature in the swarm beyond the first, add each creature's relevant damage bonus beyond the die roll (minimum of 1) to the damage of these attacks. Typically, this will be the ability modifier used to make the attack, though some features may add more to this bonus damage (like a necromancer's undead thrall).
For example, a ogre's greatclub attack deals 2d8+4 bludgeoning damage, an average of 13 damage. A swarm of 3 ogre's would instead deal 2d8+12 bludgeoning damage when using this attack, an average of 21 damage.
You can add this bonus to damage that you wouldn't ordinarily add the ability modifier to, such as most spells, or a flying snake's bite attack. If the attack or effect has multiple damage types, this bonus damage is only added to one of those damage types (one of your choice).
Why Not Full Damage?
From a narrative perspective, each creature in the swarm might be doing something different - attacking from behind cover of allies, helping others, defending themselves, fumbling, or missing, all factors that reducing the damage dealt by the swarm.
Mechanically, large numbers of creatures can easily replace many player characters, and take a long time to manage. By reducing the damage dealt, damage focused player characters still have value. By using the ability score instead of rolling, on the fly damage is easily calcuated and critical hits become less unpredictable.
Targeting
When a swarm selects a target, all members of the swarm select the same target. For example, a swarm of 3 acolytes casting cure wounds does not choose 3 different targets, instead all targeting the same creature.
Miscellaneous Notes
Some abilities might need adaptation to work better with swarms. For example, a zombie's undead fortitude is not specifically covered by these rules, and might need to be addressed specifically by your DM. As these edge cases are found, they will be added to this document.
- If a swarm of zombies passes its save on undead fortitude, you still reduce the total damage taken by a zombie's maximum hit points, and note how many zombies should be in the swarm. Any further damage will force another undead fortitude save, and a failure will kill all excess zombies.
- If you have a swarm of prebuilt swarming creatures, such as a swarm of quippers, ignore the effects of killing half of the prebuilt swarm on damage until there is a single creature left in the swarm.
- You can add the damage modifier bonus to effects like sleep or cure wounds that require rolls akin to damage, but don't necessarily injure creatures.
Variant: Spellcasting Alternatives
At DM discretion, spellcasters in a swarm may choose to one of the following benefits instead of adding their ability modifier to their damaging spells:
- The spell is cast one level higher than its base level for each member of the swarm beyond the first, to a maximum of 9th level. For example, if three clerics each use a 2nd-level spell slot to cast prayer of healing, the spell is cast at 4th level (two levels higher) when the spell is completed.
- If the spell works over a certain range or creates an area of effect, one of these distances can be doubled for each member of the swarm beyond the first. For example, if seven wizards are casting locate object, the object can be sensed if it is within 64,000 feet (about 12 miles), instead of the normal 1,000-foot range.
- If the spell can be made permanent after being cast a certain number of times, as with the teleportation circle spell, each member of the swarm contributes one casting of the spell to the total. Thus, a swarm of five wizards could create a permanent teleportation circle is 73 days, instead of 365.
- The spell's duration is doubled for each member of the swarm beyond the first. For example, a swarm of four druids could increase the duration of a polymorph spell to 8 hours, instead of one.
Finally, if you have a swarm of spellcasters or a follower concentrating on a spell effecting a creature engaged in initiatve, that spellcaster must also be engaged in initiative or lose concentration.
Exceptions for Complex Followers
Some followers might be more complicated to command than others - for example, you might conjure a Couatl or Hag, or a Giant might accompany you and take your orders.
Since these creatures have a larger number of decisions to make compared to a zombie or similarly simple creature, your DM might rule that such creatures take up more than one swarm slot, up to all 3. This is so that play is able to continue without waiting for a single player to choose everything that their followers will do each turn.
Special thanks to u/kcon1528 for group spell casting rules.