How do vulnerabilities combine, when taking multiple types of damage?

Question

When a creature has multiple vulnerabilities, and is hit by an attack that deals more than one type of damage that it is vulnerable to, do the vulnerability effects combine?

Example 1: Creature is Vulnerable 5 Thunder and Vulnerable 2 Lightning. Attack deals Thunder and Lightning damage. Does the creature take 7 extra damage?

Example 2: Creature is subjected to a single effect that deals ongoing Fire and Radiant damage, while it is Vulnerable 5 All. Does the creature take 10 extra damage, when taking the ongoing damage?

Answer 1

If I'm not mistaken, your Example 2 actually encompasses two different cases, so I'm restating your examples and breaking out my answers accordingly:

Example 1: Creature is Vulnerable 5 Thunder and Vulnerable 2 Lightning. [A single] Attack deals Thunder and Lightning damage.

Example 2a: Creature is affected by [A single effect dealing] Ongoing Fire & Radiant 5, while it is Vulnerable 5 All.

Example 2b: Creature is affected by [two separate effects dealing] Ongoing Fire 5 and Ongoing Radiant 5, while it is Vulnerable 5 All.

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Which should result in the following rulings:

Outcome 1: 7 extra damage.
Outcome 2a: 5 extra damage.
Outcome 2b: 10 extra damage.

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Assumptions:

• "Vuln [type] [#]" means "whenever the creature suffers a hit of damage type [type], the creature takes [#] extra points of damage."
• Whenever a creature suffers a discrete incident of damage, evaluate that damage in isolation against all the creature's vulnerabilities, resistances, triggering effects, etc. etc.
• If a single damaging effect has two types, all points of damage are considered simultaneously to be of both types; 5 fire&radiant is not 2.5 fire / 2.5 radiant, and thus is not two separate 'hits.'

Conclusions:

• In example 1, the creature suffers a single instance of damage which just happens to deal Thunder and Lightning damage. Evaluate the hit against all the creature's vulnerabilities:
  1. The creature's Vuln 5 Thunder triggers (since the damage is typed Thunder, it meets the criteria for this vulnerability).
  2. The creature's Vuln 2 Lightning triggers (also meets the criteria for this vulnerability).
• In example 2a, the creature suffers a single instance of damage which just happens to count as both Fire and Radiant simultaneously. Therefore the "Vulnerable 5 All" gets triggered once (the typing of the damage [Fire&Radiant] meets the criteria for the vulnerability).
• In example 2b, the creature suffers a separate instance of Fire damage and Radiant damage - two separate "hits" trigger the Vulnerable rules two separate times, exactly as if two separate instances of example 2a had occurred.

Answer 2

I don't think there's a clear ruling on this anywhere, although I'd be happy to be proven wrong. For reference, the Player's Handbook errata includes this note:

"Vulnerability to a specific damage type applies even when that damage type is combined with another. For example, if you have vulnerable 5 fire, you take 5 extra damage when you take ongoing fire and radiant damage."

The Rules Compendium preview gave us this text: "Vulnerabilities to the same damage type are not cumulative." Note that it's calling out "same damage type" explicitly, rather than generally stating that vulnerabilities aren't cumulative.

From that, I would conclude that when an attack does multiple damage types and the target is vulnerable to both, it generally stacks. I.e., in example 1, the creature would take 7 extra damage. However, I wouldn't object to a ruling the other way. See also this thread.

Vulnerable X all is probably a special case; it means that when you take any damage, you take X more points. You don't need to split that into damage types when multiple damage types apply, because it's all-encompassing. This is more subject to argument than the above case, however, and I can't find any cites at all for it.

Answer 3

(Thunder and Lightning) is considered one damage type. If you have vulnerable to thunder and/or lightning, you have vulnerable to (Thunder and Lightning). Use the higher value.

(Fire and Radiant) is considered one damage type. If you have vulnerable to all, you have vulnerable to (Fire and Radiant). Add the vulnerable modifier once whenever you take this damage type.

Use the highest applicable vulnerability and/or the lowest resist value.

Answer 4

Example 1: 7 extra damage.

Example 2: It just takes 5 extra damage.

I'm looking for the rules cites. Well, the main part on the rules on Resistances and Vulnerabilites in the PHB is on page 276. But it doesn't explain this very well.

Vulnerability against the same damage type does not stack (So if you have Vuln 5 to Psychic and you get hit with a Vuln 10 Psychic type effect, you just change to 10, not 15). But the vulnerabilities are applied separately when they are different damage tpes: If it's a Thunder/Lightning attack and you are vulnerable to both thunder and lightning, you take the extra damage from both.

Answer 5

Assumption #0: Vulnerability and resistance are 2 sides of the same coin and have a consistent application of concepts. Understanding one is at least instructive to adjudicating the other.

Assumption #1: For all resistances an vulnerabilities, the highest numerical value applies for any given damage type. Derived from "Not Cumulative", Rules Compendium (RC) pg. 224-226

Assumption #2: When determining damage, the damage is adjusted by the least resistance applicable and by the greatest vulnerability applicable. Derived from "Against Combined Damage Types", RC pg. 224-225 (Basically the damage always flows to where it will do the most damage.)

Assumption #3: Resistance or Vulnerability to "ALL" damage equates to a specific resistance/vulnerability to each damage type individually, as well as a specific resistance/vulnerability to untyped damage. Derived from "Combined with Resistance" pg. 226.

This last example under "Combined with Resistance" is particularly telling since the damage type is capable of activating each vulnerability by itself, but the damage is only modified by the greater of the two.

Skeolan has a well reasoned response to this question, but I have a objection to his second assumption. If the discrete damage was applied to each vulnerability in isolation, then the example on page 226 would result in 7 extra damage, not 5 damage as it describes in the RC. The application of multiple vulnerabilities stacking relies upon this assumption. In his defense, he answered before the Rules Companion was widely available, so he did not have the benefit of that change/example.

The interpretation of the rules that has the most consistency is that all vulnerabilities are considered in aggregate, not isolation and the damage is applied only once to the greatest applicable vulnerability.

This also mirrors the specific example of multiple damage types vs multiple resistance types under "Against Combined Damage Types" on page 224-225. Basically, resisting both types of damage dealt does not activate both resistances or provide additional protection the attack. Only the lesser resistance counts. In a similar way, being vulnerable to both types does not activate both vulnerabilities to make you more vulnerable, only the greater vulnerability counts.

Damage seeks to do the most damage, but each discrete instance of damage is only applied to one resistance and one vulnerability.

So to answer the original question (borrowing from Skeolan's breakout of case 2):

Outcome 1: 5 extra damage

Outcome 2a: 5 extra damage

Outcome 2b: 10 extra damage

Answer 6

The recent Ampersand article http://www.wizards.com/DnD/Article.aspx?x=dnd/dramp/20100813 contains an except from the upcoming rules compendium, and answers this question.

Against Combined Damage Types

Vulnerability to a specific damage type applies even when that damage type is combined with another. For instance, if a creature has vulnerable 5 fire, the creature takes 5 extra fire damage when it takes ongoing fire and radiant damage.

Not Cumulative

Vulnerabilities to the same damage type are not cumulative. Only the highest vulnerability applies. Example: If a creature has vulnerable 5 psychic and then gains vulnerable 10 psychic, it has vulnerable 10 psychic, not vulnerable 15 psychic. Similarly, if a creature has vulnerable 5 psychic and then gains vulnerable 2 to all damage, the creature still has vulnerable 5 psychic, not vulnerable 7 psychic