First, I've added in a druid orison: Deep Woods Off. It's got a distance of touch, a duration of 1 hour and anyone under its effect will not be attacked by any vermin swarm in which they find themselves, as long as that swarm is made up of fine vermin.
Second, I've ruled that the druid himself is always immune to the deleterious effects of his own swarm.
Third, I've created a Wasp Swarm monster by starting with a Spider Swarm, removing the poison damage and climb speed, and adding in a fly speed of 30. (I figure that with the poison damage AND a fly speed, it becomes too powerful).
Fourth, I've ruled that since they're mindless, they essentially get two tricks: Swarm and Settle Down. When they swarm, they form a normal 10' cube swarm, with the druid in one corner of it and otherwise swarming to attack as many living critters as they can (allies or enemies, doesn't matter ot them). When they settle down, they all crawl back into the druid's clothing.
I set it up this way because if the swarm was directable, it could become an extremely vicious attack: unless the enemy had some way of doing area damage, the enemy would have no chance of surviving the attack. If the swarm stays around the druid, on the other hand, then the druid himself has to enter danger in order to get the wasps to do so.
Does this sound balanced to folks? As it is, I think this swarm has been less useful in fights than, say, a wolf would be: the druid is uncomfortable bringing it out in fights wherein he's standing next to his friends, since it'll hurt them too. But if I make it directable, then it's basically like a wolf that doesn't trip but that always hits, cannot be hit itself, disrupts spellcasting, and often nauseates enemies.
Daniel
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