I've heard this coming up more and more, and debated the rule all over, including with the guys of Forge the Narrative, the 11th Company, and Team Stomping Grounds. Obviously, there is debate and the answer isn't abundantly clear, but I'm here to settle it.
An Iron Priest on Thunderwolf Mount hits at S10.
Now, let's look at why:
The confusion stems from the first couple pages of the rule book where Modifiers and Multiple Modifiers are discussed:
If a model has a combination of rules or wargear that modify a characteristic, first apply any multipliers, then apply any additions or subtractions, and finally apply any set values. For example, if a model with Strength 4 has both'+1 Strength' and 'double Strength', its final Strength is 9 (4x2=8, 8+1=9). If a model with Strength 4 has both '+1 Strength and 'Strength 8', its final Strength is 8 (ignore +1 Strength and set it at 8).This sets the order of operations for multiple modifiers, and is largely the source of confusion when it comes to Thunderwolf mounts. The argument partly lies on the fact that Thunderwolf mounts are wargear, which is mentioned in the Multiple Modifiers rule.
However, this rule of Multiple Modifiers does not apply to Thunderwolf Mounts.
To understand why, we have to look at how the Rules define modifiers. Specifically, a modifier to a statistic users a mathematical symbol to define what modification it has. This is fairly obvious with melee weapons, which often have a S value of +1 or x2. This is also seen in spells, particularly those from Biomancy, like Iron Arm and Enfeeble:
Iron Arm is a blessing that targets the Psyker. Whilst the power is in effect, the Psyker has +3 to his Strength and Toughness and he gains the Smash special rule.With the Multiple Modifiers rule, a Librarian who managed to get a hold of a thunder hammer, and then cast Iron Arm, would have Strength 11 (4x2=8, 8+3=11), which is modified down to 10 as a max characteristic. A S2 Grot that somehow became a psyker, cast Iron Arm, and had a power claw, would be S7 (2x2=4, 4+3=7). Finally, a Space Marine with a Power Fist who has Enfeeble cast on him, would strike at S7 (4*2=8, 8-1=7).
Now, the reason I'm belaboring the math and the point that modifiers have math symbols like "+" or "x" is because the entry for Thunderwolf Mounts does not include these symbols:
Models with a Thunderwolf mount change their unit type to Cavalry, as described in Warhammer 40,000: The Rules. All close combat attacks made by a model with a Thunderwolf mount have the Rending special rule. In addition, a model upgraded to have a Thunderwolf mount increases* their Strength, Toughness, Attacks and Wounds characteristics by 1 (these bonuses are already included in the profiles of models that have a Thunderwolf mount as part of their standard wargear). (*emphasis added)The key part here is that the statistics are increased by 1, not given +1. This distinction is important, and can be seen with Bikes as well. The exact same wording is in the main Rulebook for Bikes and Jetbikes:
Bike and Jetbike riders benefit from an increase* to their Toughness characteristic by 1. If the Bike or Jetbike is part of the model's standard wargear, this bonus is already included on its profile. (*emphasis added)The parallel to Bikes is key, because it grounds us with a familiar statistic increase.
Thunderwolves are particularly problematic because of the number of rules that interact with Strength compared to the few that interact with Toughness. The main Toughness concern is Instant Death, which includes a caveat in the rule that says it applies after modifiers.
On the other hand, there are a couple obvious areas where Strength being increased or modified is a concern.
To start, let's look at Hammer of Wrath, which "is resolved at the model's unmodified Strength." If Thunderwolf Mounts are a modifier, characters with a Thunderwolf mount have S4 Hammer of Wrath hits.
Additionally, with that ruling, would you then subtract the bonus point of Strength included in the profile of Thunderwolf Cavalry for their Hammer of Wrath? Playing devils advocate, while the profile is S5, the bonus from Thunderwolf mount, which is an item of Wargear in the unit entry, is already included, as indicated in the Wargear entry. However, there is no precedent for modifying a listed characteristic to get an unmodified value. That is clearly counter to reason. Therefore, the increase in Strength as included in the profile is not a modifier. By extension, there is no reason why the increase would be a modifier for some models and not for others that have the "wargear" standard.
Finally, getting to the root question; if a model on a Thunderwolf mount has a weapon that doubles strength, is it subject to multiple modifiers?
My answer is no, it is not. A Thunderwolf Cavalry Pack Leader with a Power Fist unquestionably is S10; his base profile lists his base Strength as 5 because a Thunderwolf mount is in his standard wargear. This item of wargear is in no way different to that purchased by an Iron Priest or other Character with the option. Therefore, there is no reason it should interact with other items of wargear differently. Because the bonuses are increases and not modifiers, they should be included in the base profile.
In the end, it is confusing and not quite as clear cut as I would like. However, evidence of the wording on modifiers compared to the wording for Bikes and Thunderwolf mounts indicates they are not the same. Therefore, the Multiple Modifiers rule does not apply to Characters with Thunderwolf Mounts and Weapons that double Strength.
So, do you agree? If not, what evidence am I missing that points to a different conclusion?
Also, I am considering making this a semi-regular segment. If so, what rules debates haunt you? What rules do you think need research and clarification?