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AFAIK, there are no rules for making a swarm. I believe that you should sit down with the base monster, look it over, decide on the CR you want the swarm to have and then use that CR as a design guideline for creating the swarm, comparing its abilities to other swarms. I can see any monster becoming a relatively ugly swarm depending on the desired CR.
As an example, I could create an "ant" swarm, which sounds fairly benign. This ant swarm would have the same stats as a spider swarm, possibly using a different poison. But if I think "army ants" and make the damage 2d12 a round and add in the Scent Special Quality, add a Special Attack that makes spellcasting impossible while in the swarm and then give the swarm a sort of collective intelligence that can be used in combat or used to overcome obstacles in some fashion, the CR just got much higher.
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Don't reinvent the wheel. Take a look at what exists and call it something different and maybe modify it slightly to suit your needs. Here is a list of swarms that I know of...
Crawling Claw -- Lost Empires of Faerun
Swarm, Apocalypse Frog -- Book of Exalted Deeds
Swarm, Bronze Locust -- Book of Exalted Deeds
Swarm, Deathraven -- Book of Exalted Deeds
Swarm, Sunfly -- Book of Exalted Deeds
Swarm, Pest -- Cityscape
Ectoplasmic Swarm -- Complete Psionic
Hoard Scarab Swarm -- Draconomicon
Verx Swarm -- Dragon 281
Golem Swarm -- Dragon 309
Prismfly Swarm -- Dragon 321
Nerve Swimmers -- Dragon 337
Ettercap Brood Swarm -- Dragon 343
Seedroach Swarm -- Dragon 355
Draconic Fingerlings -- Dungeon 102
Hetfish -- Dungeon 112
Bonespider Swarm -- Dungeon 119
Flying Fingers -- Dungeon 127
Wormswarm -- Dungeon 130
Hastendeath Spider Swarm -- Dungeon 138
Ruin Swarm -- Epic Level Handbook
Aquatic Ooze, Bloodbloater -- Fiend Folio
Swarm, Abyssal Ant -- Fiend Folio
Swarm, Average Pack Cranium Rat -- Fiend Folio
Swarm, Bloodfiend Locust -- Fiend Folio
Swarm, Greater Pack Cranium Rat -- Fiend Folio
Swarm, Lesser Pack Cranium Rat -- Fiend Folio
Swarm, Plague Ant -- Fiend Folio
Swarm, Rapture Locust -- Fiend Folio
Swarm, Scarab Beetle -- Fiend Folio
Swarm, Viper -- Fiend Folio
Swarm, Wasp -- Fiend Folio
Broodswarm -- Hordes of the Abyss
Bloodmote Cloud -- Libram Mortis
Bone Rat Swarm -- Libram Mortis
Corpse Rat Swarm -- Libram Mortis
Silthilar -- Lords of Madness
Swarm, Bat -- Monster Manual
Swarm, Centipede -- Monster Manual
Swarm, Hellwasp -- Monster Manual
Swarm, Locust -- Monster Manual
Swarm, Rat -- Monster Manual
Swarm, Spider -- Monster Manual
Brood Keeper Larva Swarm -- Monster Manual 3
Cinder Swarm -- Monster Manual 3
Dread Blossom Swarm -- Monster Manual 3
Ephemeral Swarm -- Monster Manual 3
Needletooth Swarm -- Monster Manual 3
Shimmerling Swarm -- Monster Manual 3
Shredstorm -- Monster Manual 3
Swamp Strider Swarm -- Monster Manual 3
Clockwork Mender Swarm -- Monster Manual 4
Demonet Swarm -- Monster Manual 4
Mageripper Swarm -- Monster Manual 4
Tomb Spider Broodswarm -- Monster Manual 4
Elementite Swarm, Air -- Planar Handbook
Elementite Swarm, Earth -- Planar Handbook
Elementite Swarm, Water -- Planar Handbook
Scarab Swarm, Death -- Sandstorm
Scorpion Swarm -- Sandstorm
Feral Spirit -- Sharn: City of Towers
Rancid Beetle Swarm -- Sharn: City of Towers
Swarm, Jellyfish -- Stormwrack
Swarm, Leech -- Stormwrack
Swarm, Piranha -- Stormwrack
Devil, Ayperobos Swarm -- Tyrants of the Nine Hells
All-Consuming Hunger -- Underdark
Director of Narrative
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A while back I did a nasty stirge swarm. I still have my notes on it somewhere, but essentially what I did was take the statblock of the base critter and the swarm statblock and found the common denominators. Then I reversed the process with the statblock for the stirge. I wasn't totally happy with what I came up with and I know there could be some improvements. (An edited version will be showing up in the next Oerth Journal.)
So unless you're super-frisky for some crunching, take critter-master Moff's advice and keep it simple.
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My problem with the swarm subtype is the ambiguity of the rules. Why does a swarm of bats have 3 HD while a swarm of centipedes has 9 HD? Why does the swarm get the size modifier to AC based on the individual creature? Shouldn't a swarm be easy to hit with a grenade-like weapon? Take a spider swarm as an example. It has an AC of 17 (+4 size, +3 dex) and 9 hp (2d8 HD). Wouldn't it be more appropriate to lower its AC to 12 (-1 size, +3 dex) and give it more HD, say 4d8 (18 hp)?
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My problem with the swarm subtype is the ambiguity of the rules. Why does a swarm of bats have 3 HD while a swarm of centipedes has 9 HD? Why does the swarm get the size modifier to AC based on the individual creature? Shouldn't a swarm be easy to hit with a grenade-like weapon? Take a spider swarm as an example. It has an AC of 17 (+4 size, +3 dex) and 9 hp (2d8 HD). Wouldn't it be more appropriate to lower its AC to 12 (-1 size, +3 dex) and give it more HD, say 4d8 (18 hp)?
What good is the AC to a spider swarm? It is immune to damage. The only thing that really affects it is AoE damage -- in which case AC is irrelevant.
I don't know about the HD issue. You might just think about it more in terms of how much damage the swarm can take before it disperses and moves on. Maybe bats are smart enough to move on after 3 HD worth of damage. Just guessing here.
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What good is the AC to a spider swarm? It is immune to damage. The only thing that really affects it is AoE damage -- in which case AC is irrelevant.I don't know about the HD issue. You might just think about it more in terms of how much damage the swarm can take before it disperses and moves on. Maybe bats are smart enough to move on after 3 HD worth of damage. Just guessing here.
The rules seem a bit fuzzy on this and contradict themselves a few times:
Player's Handbook, page 158: To attack with a splash weapon, make a ranged touch attack against the target. A hit deals direct damage to the target, and splash damage to all creatures within 5 ft. of the target.
You can instead target a specified grid intersection. Treat this as a ranged attack against AC 5. However, if you target a grid intersection, creatures in all adjacent squares are dealt the splash damage, and the direct hit damage is not dealt to any creature.
Monstrous Manual, page 237: A lit lantern can be used as a thrown weapon, dealing 1d4 points of fire damage to all creatures in squares adjacent to where it breaks.
Monstrous Manual, page 316: A swarm takes half again as much damage (+50%) from spells or effects that affect an area, such as splash weapons and many evocation spells.
Maybe I'm way off in my understanding of the rules, but using AoE items such as flasks of acid and alchemist fire still means you have to get that touch AC. If that CR 1 spider swarm has a touch AC of 17, that is far from an easy hit. If you miss that swarm with the flask or decide to aim for a grid intersection (AC 5), you only deal 1 point of damage (+50%). However, according to page 237 in the Monstrous Manual, a lit lantern deals more damage than alchemist fire. Huh? This is where I get confused.
- Does a lit lantern thrown at a grid intersection deal 1 damage,1d4 damage, or 1d4+50% to all adjacent squares? Does the lantern have a 50% of not igniting like a flask of oil? What about the lantern itself? Is it reuseable after the attack? Does it make a difference whether you're throwing a bullseye or hooded lantern?
- What about a flask of alchemist fire? If you aim it at the swarm do you need to get that AC 17 or AC 5? If you aim it at a grid intersection, will the flask deal 1, 1d6, or 1d6+50% damage?
I like the idea of swarms, but I wish whoever came up with them had explained things a bit clearer. I'm hoping someone on these boards has the answers I'm looking for.
I would assume that the lantern would have to break open in order to spread the lit oil to do damage. Further, alchemist's fire does an additional 1d6 points of damage on the second round, whereas the lantern seems to only do damage on the first round.
That having been said, it would make more sense to be and seem to be more consistant to say that the lantern does 1d4 on a direct hit, 1 on a splash hit or if its thrown at a square instead of at the creature in question.
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Maybe I'm way off in my understanding of the rules, but using AoE items such as flasks of acid and alchemist fire still means you have to get that touch AC. If that CR 1 spider swarm has a touch AC of 17, that is far from an easy hit. If you miss that swarm with the flask or decide to aim for a grid intersection (AC 5), you only deal 1 point of damage (+50%). However, according to page 237 in the Monstrous Manual, a lit lantern deals more damage than alchemist fire. Huh? This is where I get confused.
- Does a lit lantern thrown at a grid intersection deal 1 damage,1d4 damage, or 1d4+50% to all adjacent squares? Does the lantern have a 50% of not igniting like a flask of oil? What about the lantern itself? Is it reuseable after the attack? Does it make a difference whether you're throwing a bullseye or hooded lantern?
- What about a flask of alchemist fire? If you aim it at the swarm do you need to get that AC 17 or AC 5? If you aim it at a grid intersection, will the flask deal 1, 1d6, or 1d6+50% damage?
I understand what you are saying and you bring up some good points. This is how I would deal with it...
For creatures that are immune to damage from weapons and such, the AC is there if needed to actually hit one of the creatures in the swarm. Most of the time this is silly as you are not trying to hit one creature but the swarm as a unit. For this reason I would generally say that you were trying to hit the square that the swarm is found in. Especially since alchemists fire and lanterns actually have to break and I don't think that the body of a spider is sufficient to break a flask or a lantern.
Lantern -- I would say that it does 1d4 +50% damage to the square and all adjacent squares. The problem is that it is one swarm, so the adjacent squares really don't matter since they are not separate "creatures" so it would only be 1d4+50% total damage to the swarm (unless there was an additional swarm in an adjacent square). The description says "where it breaks", so, no, you cannot use the lantern again. I don't know if it matters what kind of lantern is used.
Alchemists fire -- I would say that it does 1d6 +50% damage to the square. It could do 1 point of splash damage to an adjacent swarm if there was one. As far as doing an additional 1d6 +50% damage the next round, I would say that this is only true if the swarm remains in the area that was hit (or 1 point in adjacent squares). The swarm doesn't catch fire because it automatically kills all the creatures that are on fire, so the fire doesn't move with them. The fire does stay on the ground and so if the swarm moves through the square it would take damage.
I hope that this helps.