Hey, I just had a question about the Body of Ice ability of the Ice cleric subdomain. Namely, does the extra damage taken from fire stack with vulnerability to fire from other sources? In particular, for instance, if a creature of the cold subtype (which therefore had vulnerability to fire) had this ability due to cleric class levels, would it therefore while in ice form take three times normal damage from fire? (x2 due to Body of Ice, x1.5 due to fire vulnerability) Or does the higher vulnerability take precedence, so the damage would still be only x2?
Yes, they stack.
Unfortunately, the ability says you take double damage from fire. It doesn't not call it a vulnerability, so we have to assume this is something similar to, but different from vulnerability. If it had said, you gain vulnerability to fire that causes double damage, then they would not stack.
Multiplying
When you are asked to apply more than one multiplier to a roll, the multipliers are not multiplied by one another. Instead, you combine them into a single multiplier, with each extra multiple adding 1 less than its value to the first multiple. For example, if you are asked to apply a ×2 multiplier twice, the result would be ×3, not ×4.
So a fire vulnerable creature using body of ice would take 1.5 + (2 - 1) = 2.5 times more damage, not 3.
I know that's normally how multipliers combine, but I wasn't sure whether this case would be different. Though, really, I can't think of any good reason this case would be different, so you're probably right.
Your argument that it isn't called a vulnerability and therefore should stack with it is a good one... that's pretty much what I'd been thinking too, but I wasn't sure it was right.
Thanks for the reply.