Well, a slight quibble, but "immune to weapon damage" arguably negates the extra fire damage dealt by a
flaming weapon or a torch - if so, your supposed counter doesn't work. Even if the "immune to fire damage" thing doesn't cover it,
flaming weapons and torches only affect a discrete number of targets, and that also makes a swarm immune to them. But I'll leave that aside since it's not perfectly solid ground to argue from.
Hmm... I still think your example is not much good, but it'll be more productive to start over. I think we can all agree that the RAW doesn't have a black-and-white answer to whether or not a swarm can be prone or not. We're both extrapolating from what's given to what we think is reasonable. I think it makes sense that a swarm can suffer the effects of being prone in unusual circumstance, and I think that
grease is such a circumstance. You think it's ridiculous to apply the effects of being prone to a swarm.
I'll assume that you think greasing a swarm of cats would be effective, since greasing a whole bunch of cats makes about as much sense as greasing a single cat (and we know that's possible). I hope you don't think the idea of the cats moving on top of each other to form a pontoon bridge across the grease is very plausible, especially since 300 cats in a 10-foot square doesn't make for very much depth of feline. The pressing issue is the centipede swarm and others like it.
Let's check out exactly what those effects are.
SRD said:
Prone: The character is on the ground. An attacker who is prone has a –4 penalty on melee attack rolls and cannot use a ranged weapon (except for a crossbow). A defender who is prone gains a +4 bonus to Armor Class against ranged attacks, but takes a –4 penalty to AC against melee attacks.
Standing up is a move-equivalent action that provokes an attack of opportunity.
Lets go through these effects one by one and see if a reasonable interpretation of the
grease spell would allow them.
1) "The creature is on the ground."
That's borderline flavor text, to my mind. I'm okay with stipulating that the swarm aggregate has a somewhat lower profile when prone than when not prone - the bottom row of critters (not that they travel stacked on one another like legos anyway, but...) is churning and trying to keep its footing, so the whole mass is a little lower to the ground than usual.
2) "An attacker who is prone has a –4 penalty on melee attack rolls."
Swarms never make melee attack rolls, so this one's irrelevant.
3) "and cannot use a ranged weapon (except for a crossbow)."
Swarms never use weapons of any kind, as far as I know. Some of them have natural ranged attacks, true, but that's not a ranged weapon so this restriction does not apply.
4) "A defender who is prone gains a +4 bonus to Armor Class against ranged attacks"
Now here's a quirky one. How to justify a +4 bonus to a
greased swarm's armor class against ranged attacks? Well, a bunch of slippery cats is liable to be harder to peg with an arrow, I suppose, and a smaller swarm doesn't care about ranged attacks anyway. Even stuff like
polar rays, I might add, since ray spells only affect a discrete number of targets. So this effect of being prone is either irrelevant or doesn't pose a logical problem (in the greased kittens case).
5) "but takes a –4 penalty to AC against melee attacks."
Again, this one may or may not be relevant. I'll assume for the sake of argument that a character's chance to hit a swarm with a
flaming weapon actually matters. To me, this looks like a natural effect of a swarm trying to "pull itself together" - it's not like the swarm travels stacked neatly in rows, after all. The bottom-most centipedes are going to be slipping around in the
grease spell and creating confusion for the top-most centipedes when the whole group "wants" to move as a whole. But those topmost centipedes that are already in a characters clothes and hair aren't really going to be affected - they'll keep chewing away independently and dealing swarm damage unless the whole swarm decides to move away as a whole, at which time the fact that the creatures that are actually touching the ground and providing impetus for the swarm's movement are unable to get traction and are creating a lot of confusion in the lower swarm-ranks becomes a pressing issue. Which takes us to the next pronely condition...
6) "Standing up is a move-equivalent action that provokes an attack of opportunity."
Well, for starters we know that a swarm can provoke attacks of opportunity when it moves to do something - all swarms automatically provoke and AoO when they move into your square, it's in their creature subtype description.
So let's envision the situation on a failed Reflex save (if the swarm makes its save, what are we arguing about, after all?). The centipedes closest to the bottom are having some real difficulties - they can't get a footing, and when they try to go up higher they just push down more centipedes that can't do anything more than pedal their little legs ineffectually. The whole mass decides to start moving forward, but darn it those under-centipedes are confusing the whole issue with their flailing and their inability to move along the ground. So the swarm as a whole takes a second or two to sort things out, and leaves itself vulnerable for a bit as it reorganizes itself to get across the
grease. This provokes an attack of opportunity. Makes sense - the swarm has to fumble around for a bit to find the "configuration" that gets it across the slime. It's just (literally) for a couple of seconds, and the swarm doesn't really care about the attack of opportunity since it's immune to weapon damage.
A couple of seconds of fumbling later, the swarm has "figured out" a way across the slime and moves out of the area to continue its nibblings elsewhere.
Now, to me there's nothing ludicrous about this example or any of the effects of being prone as applied to the swarm. I suppose you might think differently, and I can see that too, but please refrain from saying that the "
grease affects a swarm" position is completely nonsensical. Remember in my first post I said that I would make the call depending on which option led to the most interesting situation, and then stick to that call for the rest of the campaign. I still think that's the best solution, since both the pro-
grease and anti-
grease positions are pretty intelligible.
And now for the last bit...
KarinsDad said:
Where exactly does the Grease spell indicate how thick it is?
It doesn't, it just says that it produces "a layer." You could take that to mean anything from a molecule-thin coating of teflon to a good solid inch of bacon grease. I've always taken it to be the later, because it's a cooler visual and because the spell's material component is "a bit of pork rind or butter." But you could call it either way, really.