Shattered Kingdoms

Do they stack at all? For instance, would a deep-elf hellion get hit for double damage from a BoG?

They do not stack, only the most powerful vulnerability applies. A deep-elf hellion will take just as much raw damage from BoG as a deep-elf of any class, whereas a human hellion will actually take less damage from BoG than a deep-elf of any class. This introduces a funny mechanism where deep-elf casters can get 2-3 shotted by BoG sometimes, whereas sprite necros with decent enchantments can often laugh BoG's off. In the same way, a sprite will take just as much damage from a cold iron blunt weapon as from an either blunt or cold-iron weapon of the same subtype and enchantments, excluding ironguard of course. I also believe that weaknesses and vulnerabilities also apply at the same time: A centaur with resist elements up will still take more damage from a fire-based attack than a non-centaur with RE up.

Weapons are a little different. Weapons with a magical attack or made out of a magical material end up having their damage split amongst their material type and their attack type. A steel halberd with a flaming bite, for example, does 50% physical steel damage and 50% fire damage with each swing. As a result, a centaur with ironguard up will take reduced damage from the steel portion of each attack but will still take increased damage from the fire portion of the attack and it will just about even out to the same damage, even though he'll still get the "xxx's weapon passes through you" message.

Hrm, this is new information for me. Thank you, Eddy.

I'm pretty sure it's true at least

Thank you Edoras.

Classes generally don't have any energy weaknesses, this is only a function of race. So there is nothing to stack or overlap in the first place.

Hellions and necros, who must be evil, will take additional damage from some holy spells. But this is generally due to alignment and not class.

There may be some exception I am forgetting.

Never knew there was a class weakness.

Yeah there are no class weaknesses, only a difference in class alignments and the spells that can be used on them. Hellions, necromancers, and dark-aura priests don't take any extra damage from BoG, it's just that nobody else can be hit by a BoG in the first place.