Incorporeal creatures take half damage from magic spells; swarms take 50% more damage from area of effect spells.
If you have an incorporeal swarm, how do these stack? Does each modify the original damage (so they basically negate each other) or do they stack (doing 75% damage)?
Both are applied after damage is rolled, so there's a starting point.
Hypothetically it could multiply by 1.5 and then halve, or halve and then multiply by 1.5. Either way it comes out to .75x damage.
Realistically, this would never come up, because nobody would create this creature. Because nobody would have fun fighting this creature.
Incorporeal swarm... Ick. Two of my «favorite» things to deal with indeed. It could only be worse by being a swarm of incorporeal golems xD
And yes, I aggree that it should be 75% damage.
I'm going to disagree with the consensus here. By RAW:
When you multiply damage more than once, each multiplier works off the original, unmultiplied damage. So if you are asked to double the damage twice, the end result is three times the normal damage.
So if you get two +100% multipliers, you get a net +200% multiplier. (Whereas if you were multiplying normally, you'd get x4 damage, a +300% multiplier.)
In this case the two multipliers are +50% and -50%. Each works off the original damage. They cancel out.
...unless division by two isn't the same as multiplying by 0.5 in Pathfinder logic. If it is, then two effects that cause you to take half damage would cause you to take zero damage.
Both are applied after damage is rolled, so there's a starting point.
Hypothetically it could multiply by 1.5 and then halve, or halve and then multiply by 1.5. Either way it comes out to .75x damage.
Realistically, this would never come up, because nobody would create this creature. Because nobody would have fun fighting this creature.
Already created: Shadow Rat Swarm
Oh, and before you say...but that is third party! Paizo used them in one of their APs.
Realistically, this would never come up, because nobody would create this creature. Because nobody would have fun fighting this creature.
For a martial character, an ethereal swarm of tiny creatures is no worse than a regular swarm of diminutive creatures... I think.
The shadow rat swarm is a good example of a case where damage might be halved twice. (Once for magic sword versus ethereal, and once for sword versus Tiny swarm.) I wouldn't like the result to be zero damage. So I withdraw my initial assertion. I think the additive multiplication rule is intended to only apply to positive multipliers.