Swarm (1)

Because rabid animals and nonintelligent fungi weren't quite unconventional enough PCs yet. The DM, too, shall suffer unto the locust!

General swarm rules are quite simple: they must all move into an opponent's space to attack, but are fortunately often immune or resistant to weapon attacks. There's also a few bits of cheap equipment that can deal some fire damage to them, and they die (well, 'break up') at 0 HP rather than -10. Also: about every targeted spell is ineffective against them, which includes stuff like Lesser Vigor and Fly: it's even worse than spell immunity in that you can't suppress it.

Finally, all swarms have the Distraction quality, which is a passive effect that may nauseate creatures whose square you occupy. Action denial is neat, what more is there to say?

Bat Swarm

Diminutive, so immune to weapon damage, and only 3 animal RHD. Low land speed but good (40 ft.) flight. Ability scores tend to be either slightly boosted (such as wisdom and dexterity) or very low (such as strength, intelligence, and charisma). Still, a cleric thing should work I guess?

Bat swarms also have 20 ft. blindsense (kinda meh) and wounding, which is a strong contender for 'weakest ongoing debuff that's actually noticeable'. Then again, when you're immune to all weapons at 3 HD, who needs debuffs?

I'm very torn on this one. On the one hand, immunities are awesome, but the lack of clear advancement (be a druid, I guess?), horrible hit dice, inability to get buffed or healed, and awful intelligence score aren't helping much. In the end, I'll put these down as +0, though I expect people to disagree there.

Centipede Swarm

Also diminutive, but with no fly speed, worse ability scores, and a massive 9 RHD. Weak poison and a climb speed aren't gonna save this one: -0 LA for sure.

...now that I think about it, this is probably the first vermin I've covered. Kind of funny I've never come across one in more than a year.

Hellwasp Swarm

Now this is interesting! Diminutive again, but a magical beast rather than an animal and actually sapient. 12 RHD hurt, but let's take a closer look.

40 ft. fly speed, DR 10/magic, resistance to fire and some okay ability scores don't really justify the large number of hit dice, though. The poison is not really worth mentioning, and while Inhabit is a flavorful and cool ability, it's also basically a limited form of Dominate Monster/Animate Dead. Hive Mind is more of a curse than a blessing, as it either removes a swarm's normal vulnerability to mind-affecting stuff or renders you mindless.

In the end, hellwasps (isn't that a pleonasm, now that I think about it?) fail to compensate for their hit dice, even if those are relatively powerful. -0 LA once more.