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.