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I am currently GM'ing an urban based campaign and one of the plot hooks I've come up with is the city's rats getting more and more violent, attacking people on the street in broad daylight. Eventually the PCs would have to enter the sewers and fight a Vermin Lord(or something equivalent) who has been controlling the rats. So far so good.

The problem is that party is at a level (currently 10, might be 11 or 12 by the time they get to this point) where rats and rat swarms are just no challenge. The best I could come up with from the advancing monsters advice in the MM is a 6 HD dire rat, which is still only a CR 4. According to the encounter chart in the DMG the party would need to face something like a dozen of those for it to be a challenge. And from what I can see there is no set guide for advancing any swarm.

Is there any viable way to make a rat swarm challenging for higher level parties? Obviously I don't expect a single swarm to be CR 10, but if it was strong enough that 4-6 was a good encounter I'd be happy. I just don't have enough experience with creating/advancing creatures to know how strong that actually is.

Bonus second question: Are there any rules for adding swarm as a template? I think that the advanced dire rat above would make a pretty solid base for a swarm, if only I knew how to make it.

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Have you though about literally submerging the PCS of rats ? Like millions of them in a very crowded place. You could create custom rules like, they need to kill X rats per 10 minutes else they get submerged or something. They would BREATH rats in the end. They would have to find the source of the rats like a breeder. –  Saffron Jun 20 at 15:46
    
I do kind of like that idea. That was my problem with needing so many swarms for the encounter; once they get to a confined space you can only have 2-3 swarms before they take up all the available space. Making that the point would help –  D.Spetz Jun 20 at 16:04
    
usually by the time you get to 10th level your PC's scope has grown beyond a city's rat problem... –  smcg Jun 20 at 20:00

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up vote 11 down vote accepted

Enter the cranium rat swarm (the linked monster is on Wizards’ website, but I don’t know how official it is; there are official printings of it in Fiend Folio). You’ll note it’s got a base CR of ⅛ per rat. In a swarm. The swarm gets smarter and more magical the more rats are present, and they swiftly become very dangerous, casting as a fairly-powerful sorcerer. They’re a classic D&D enemy, showing up in the excellent Planescape: Torment CRPG, and their unique mechanics make them a very interesting foe to face.

You could take this further by basing your rats on the H.I.V.E.; while the actual H.I.V.E. is a theoretical-optimization build that’s inappropriate for most games, it directs you to the Hivemind rules detailed in Book of Vile Darkness, which represent mechanics quite similar to the cranium rats. You can make very powerful foes with it.

Similar to the Verminlord build that brings the H.I.V.E. together, you might consider the various options available for a swarm druid as discussed in a previous question. A city with interesting wildlife activity seems like a great place for an evil druid to take an interest in, as a potential opportunity to forcibly “return it to nature.”

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I had considered using the cranium rats as a possible mini-boss. I'll have to check out the Hivemind rules as well. And I'll definitely take a look at that swarm druid. –  D.Spetz Jun 20 at 17:20

Another possibility would be to use house-rules for a big swarm. If your player get attacked by e.g. 100 small Rats, you don't want to roll all 100 attacks - but statistics tell you, that about 5 of them will roll a natural 20 on their attack. Since the players will have massive Armor and Defenses the other 95 attacks will probably not scare them, but they will have their hands full to actually defend against all of them at once, so the 5 critical hits will hit them directly and inflict damage with a chance of infection with some ugly disease...

So you will only have one attack per 20 Rats, but this will be an automatic critical threat and ignore the armor (since they will find some place on your body which is not protected) and the damage will represent the bites and scratches of 20 rats

I like the swarm rules in Warhammer40K Black Crusade: A swarms attack cannot be parried or dodged (because there are too many), its damage is increased by 1d if there are more than 10 and by 2d if there are more than 20 and their attack value is increased by the number of enemies in the swarm divided by 5

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