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The PCs in my homebrew campaign setting are finally to the point where a lone cat is not an excessively dangerous foe, yet a clowder of cats could still worry them. However, my research efforts have failed me: I was certain there was a method of converting individual creatures into collectives, but I can't locate it.

Are there official rules for making cats (or chickens or dogs or ponies) into a swarm or something similar so I needn't run a bunch of individual creatures? Absent official rules, are there published or playtested homebrew means to do this?

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3.5e has Swarms as a general subtype of monster, applicable to any large mass of Tiny or smaller creatures, but not as a template - there's no official method to do this. However, we can infer, from the examples given, guidelines on how to stat out a swarm. Based on the statistics of the base creature and the statted swarms given:

Size/Type
The swarm retains the size/type of the base creature (including AC/Hide bonuses due to size).

HD
The swarm has some arbitrary number of hit dice chosen based on how tough you think it should be, I guess. A swarm of rats is given as 4d8, from the base creature's ¼d8; maybe a cat swarm should have 8d8, as the base cat has twice as many HD as the base rat. But bats also have ¼d8, and a bat swarm is only granted 3d8 HD... you have some leeway.

Speed
The swarm retains the speeds and methods of movement of the base creature. The listed rat swarm doesn't have the swim speed that the base rat does but that's probably an oversight.

AC
The swarm's AC is as per the base creature.

BAB
The swarm's BAB is as per its new HD and type, though it cannot grapple.

Attack
The swarm replaces the normal attack of its constituent creatures with a swarm attack, described in the Swarm subtype, dealing damage based on hit dice; the hypothetical 8HD cat swarm would do 2d6 damage. If the base creature's attack has some special property, like Disease or Poison, this is retained by the swarm attack.

Swarms cannot make attacks of opportunity.

Space/Reach
The swarm occupies a 10x10ft. space, with the caveat that it can take any contiguous shape that covers the same total area/volume - on the standard grid, meaning it can occupy any 4 contiguous 5ft squares. It has no reach (because Tiny creatures have no reach, and reach is meaningless for the swarm attack anyway).

Special Attack
Swarms gain the Distraction (Ex) special attack, where any creature that begins its turn engulfed by the swarm must make a DC 10+1/2HD+Con Mod. Fortitude saving throw or be nauseated for one round. Spellcasting, concentration, or using other skills requiring patience/concentration while engulfed by a swarm calls for a DC 20 concentration check.

Special Qualities
Retain the special qualities of the base creature. As per the Swarm subtype swarm traits, it gains the special defensive quality of only taking half damage from slashing/piercing weapons if comprised of tiny creatures or immunity to weapon damage if comprised of fine or diminutive creatures, and all the other swarm traits described there, which grant it immunity to several effects and vulnerability to some others.

Saves
As appropriate for a creature with the swarm's HD, type, and ability scores/feats.

Abilities
Retains the base creature's ability scores.

Skills
Retains the base creature's skills, including racial modifiers, feat bonuses, and size modifiers where appropriate.

Feats
Retain the base creature's feats, and potentially gains new feats as appropriate for increased Hit Dice (remember mindless creatures such as vermin never gain feats, but animals can).

Challenge Rating
Your guess is as good as anyone else's. Rather depends on how many HD you decided would be appropriate.

Alternatively, if your players really do get up in arms about extrapolating swarm characteristics from the given examples and statting your own, maybe you could just take the rat swarm and use the given rules for increasing monster HD to make it a bit tougher and call it a swarm of mangy cats, instead.

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    \$\begingroup\$ Have you tried this, and how did it go? \$\endgroup\$ – Please stop being evil Oct 5 '17 at 0:25
  • \$\begingroup\$ Not personally, it's just an inferral from the given swarms in the SRD and the rules of the swarm subtype. However, nothing involved is at all esoteric and all swarms (absent unusual special abilities, like the hellwasp possession thing) play basically the same way; you're really just dialling in how resilient/damaging the swarm is by adjusting hit dice. \$\endgroup\$ – Carcer Oct 5 '17 at 7:40
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For Pathfinder, yes, there is a swarm template. There's monkey swarms, leech swarms, sprite swarms...

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – mxyzplk Oct 4 '17 at 23:13
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    \$\begingroup\$ Have you tried this in play, and how did it go? Pathfinder is pretty similar to 3.5 but the templating much less likely to let base creatures retain stuff, in my experience, so I'd be concerned about it performing undesirably. An example where you used the template to create a new kind of swarm and it worked would help improve this answer a lot, I think. \$\endgroup\$ – Please stop being evil Oct 5 '17 at 0:23
  • \$\begingroup\$ I still think it's worth pointing out that Pathfinder's "creature swarm" template represents a swarm of tiny versions of a much larger base creature - "Creature swarms are made up not of normal bats, rats, insects, or other vermin, but of creatures that resemble larger monsters" - so to represent a cat swarm, for instance, you'd probably want to use a larger big cat as the base creature. \$\endgroup\$ – Carcer Oct 5 '17 at 8:18

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