If you suspect that your Dungeon Master has plans to cover your character with hundreds of centipedes or darken the air with a swarm of bats, you'll need to learn how to combat these creatures en masse.
A party's first encounter with a swarm is often difficult, even deadly. They require different approaches and techniques from other monsters. Fighting a swarm is quite a challenge. A wise party prepares for this eventuality.
Coping with a swarm's special attacks is difficult. A party should spread out so that there are at least one or two members who can act each time. The damage is consistent and annoying, but the distraction/nausea can be devastating.
Damage: The automatic damage caused by a swarm to creatures in its area is quite painful. It automatically bypasses many generally useful defenses, such a high Armor Class and spells such as blur or displacement. Damage reduction does apply, but since it makes a single damaging attack, a swarm is likely to cause damage to all but the highest DR.
Distraction: Creatures in the swarm's area must make a Fortitude save or be nauseated. In addition, detail work requires a Concentration check. Spellcasters, especially low-Fort save arcane casters, loathe swarms.
The swarm's special qualities make it difficult to kill. If you don't have the right tools for the job: Run!
Half Damage from Slashing and Piercing Weapons: While no one wants to fight a swarm toe-to-toe, most ranged weapons do little damage. Many special maneuvers that melee combatants enjoy are also useless against swarms. They can't be grappled, tripped, or bull rushed. Smashing and squashing the creatures in a swarm with a blunt weapon such as a club or mace is effective against those made of Tiny creatures such as rats. Unfortunately, most swarms are made of even smaller creatures. These are immune to weapon damage.
Immune to Weapon Damage: Swarms made of Diminutive and Fine creatures take no damage whatsoever from weapons. Fighters just wind up being food for swarms of miniscule critters.
Immunity to Single-Target Spells: The spellcaster isn't faring much better. Swarms are immune to any spell or effect that targets a specific number of creatures. Touch spells, ranged touch spells, and many classic, useful spells such as magic missile are completely useless.
Area effect spells are definitely the best solution to the problem of swarms. A fireball can clear out a bat swarm or rat swarm very easily.
Nonflying swarms are best dealt with by flying creatures.
Page 237 of the Monster Manual outlines some vulnerabilities of swarms:
As a general rule, if you can get away from the swarm, do so. Destroy it from a distance if you can.
Must-Haves:Gust of wind is a great spell to have on a scroll. In addition to helping with visibility on the battlefield (see Manipulating the Battlefield, Part 4), it's hand when dealing with swarms made of diminutive or fine creatures (bats, centipedes, hellwasps, locust, and more) whether or not they're flying creatures. It often can cause enough damage to break up a swarm. If your weapons are going to be useless against a swarm, you'll need something that can damage it.
Flasks of acid or alchemist's fire are wonderfully effective.
Torches or lanterns are essential tools.
Best Buys for Lower-Level Spells: Few 1st and 2nd level spells are useful against a swarm. Burning hands is effective. Levitate can get a spellcaster away from many swarms.
Wish List: Fireball, lightning bolt, and other area effect spells are the best way to deal with a swarm.
About the Author
Gwendolyn F.M. Kestrel works full-time as a game designer for Wizards of the Coast. Recent and upcoming books include d20 Past, Races of Eberron, and Planar Handbook. She simultaneously leads the lives of an avid gamer, Ph.D. student, trio of birds of prey, and a hedonistic cat.
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