How do these effects stack?

Question

Some days ago we were talking with some friends about builds and a friend of mine told us about a monk build where you got Colossal++++ unarmed damage. The main points of the build were:

• Improved Natural Attack
• Greater Mighty Wallop
• Enlarge Weapon
• Earth Hammer
• Polymorph into war troll

So he claims that this way he gets GMW damage increase first (up to colossal) then the rest kicks in (war troll, earth hammer, enlarge weapon, Improved Natural Attack) for a total of colossal++++

I know from FAQs than you can sort buffs the way that's most beneficial to you, but i can still see the size increase from the war troll must take precedence over the other buffs since you are Large sized and GMW starts increasing size from your current weapon size), and that makes it colossal+++. On second thought ir realized Improved Natural Attack is also already present when you cast GMW so it's Colossal++ at best.

Aside from that though, I am unsure about how (and if) the other effects stack. I know similar effects and effects from the same source don't stack, i also know that usually size increases don't stack, but i'm not sure "weapon damage size" is to be considered the same way of size increases. Particularly when the spells have different descriptions about how they increase this damage.

Enlarge Weapon actually increases weapon size (even though without the usual penalties), Earth hammer and improved natural attack just increase weapon damage by one step and GMW creates an aura covering your weapon that "virtually" increase your weapon's size without making it harder to handle

I have found a guide for monk builds where it suggests these spells to buff your weapon damage but it's not clear from that guide wheter you can use them at the same time or not

Answer 1

The rules are very unclear on this.

There are two questions we need to answer here:

1. Do the various effective size increases you've listed stack with each other?
2. If so, how do they stack? That is, how much damage does the resulting character actually end up dealing with their unarmed strike?

I'll tackle these one at a time, but fair warning going in: Both are a mess.

1. Do they stack?

There are two places in the rules that address stacking.

The core rules (appearing in both the PHB and the SRD) have the following to say about stacking:

Stacking

In most cases, modifiers to a given check or roll stack (combine for a cumulative effect) if they come from different sources and have different types (or no type at all), but do not stack if they have the same type or come from the same source (such as the same spell cast twice in succession). If the modifiers to a particular roll do not stack, only the best bonus and worst penalty applies. Dodge bonuses and circumstance bonuses however, do stack with one another unless otherwise specified.

It's unclear whether these rules apply - they're about "modifiers to a given check or roll." Normally, this would mean something like "enhancement bonuses to attack rolls" or "circumstance penalties to move silently checks". However, a damage roll is clearly a "roll" of some kind, and increases or decreases to your unarmed damage based on size could be considered "modifiers" if you squint.

If these rules do apply, then the increases will stack. No type is ever given for these things (e.g., they're never described as a "size bonus" or anything like that). They just say something like (using Improved Natural Attack as an example):

The damage for this natural weapon increases by one step, as if the creature’s size had increased by one category

The second set of rules that might apply appears in the Expanded Psionics Handbook, p. 56:

Same Effect More than Once in Different Strengths:

In cases when two or more similar or identical effects are operating in the same area or on the same target, but at different strengths, only the best one applies. For example, a character under the influence of both the oak body power and the iron body spell benefits only from the stronger effect (in this case, iron body). If one power or spell is dispelled or its duration runs out, the other power or spell remains in effect (assuming its duration has not yet expired).

You might have some questions about this rule, like:

• Why does this rule appear in the section on magic/psionic transparency in the XPH? Does this mean it only applies to spells and psionic powers, or does it apply to everything?
• How similar do two effects have to be to trigger this rule? The given examples, Iron Body and Oak Body do some of the same things (natural armor bonus to AC, ability score modifiers), but also some different things (iron body makes you resistant to acid and makes you count as an Iron Golem for some purposes, while oak body instead makes you resistant to cold but vulnerable to fire).

The rules provide no clear guidance on these questions. If this rule applies, it would indicate that the size stacking you're describing wouldn't work, since "increase your unarmed damage as if you were one size category larger" and "increase your unarmed damage as if you were multiple size categories larger" are pretty clearly an example of "Same Effect More than Once in Different Strengths".

2. If so, how?

Once again, there are some relevant-looking rules, but they aren't really going to solve our problem for us.

Problem 1: The table for increasing damage via size doesn't go far enough.

The rules for increasing damage via size are described in a few places (e.g., the Improved Natural Attack feat), but the most authoritative version is probably Table: Larger and Smaller Weapon Damage (also appears in the DMG). This table lets you take your base damage if you were a medium-size character, and then tells you based on that what your damage will be at a given size other than medium.

This table has two issues for you:

1. The base damage listings don't go high enough (they top out at 2d6, the base damage of a greatsword, but medium-size monks can have 2d8 or 2d10 base damage if they're high enough level)
2. The size listings only go up to colossal. As you note in the question, you have enough size increases to take you several steps past colossal.

What are you supposed to do about this? The rules don't say. You can do some educated guessing via extrapolation from that table, but you're well into the realm of house rules at that point.

Problem 2: There's actually no such thing as "colossal++++"

The core rules only address size up to colossal. There are rules for going beyond this; they're on page 99 of Draconomicon and also appear on the SRD entry for Epic Dragons. But as far as I can tell:

• They're only written to apply to dragons
• They only describe very specific benefits (e.g., they don't say anything about how they affect unarmed damage)
• They don't address how going more than one step beyond colossal works (i.e., "colossal+" is a thing, but "colossal++" and beyond aren't)

So, when you're trying to calculate the damage of a Monk with 2d10 base unarmed damage who's four size categories beyond colossal...you've moved beyond the realm of what's covered by the rules.

So how would I rule?

At the end of the day, when the rules don't clearly cover something, I tend to fall back on "what would be good for the game?"

In D&D 3.5, unarmed combat melee characters, especially monks, are notoriously weak. And, in a game where spellcasters have tricks like "take an arbitrarily large number of actions per turn" and "remove multiple enemies from a combat, no save," allowing Punchy McPunchface to do a big pile of damage with his unarmed strikes isn't going to rock the boat.

So what I would do is:

• Allow the various effects you've listed to stack in a straightforward, additive manner.
• For base damage values that aren't listed on the weapon size table (e.g., those from monk's class-based advancement), increase die size by one for each step up. So, a monk who deals 2d8 base damage when Medium would deal 8d8 base damage when colossal, and a monk who deals 2d10 base damage when medium would deal 8d10 base damage when colossal.
• For size categories beyond colossal, extrapolate from the huge > gargantuan > colossal progression, and add two dice for each size increase.

So, combining all of the above, a level 20 Monk who counted as colossal++++ would deal 16d10 base damage. That's a lot! But it's also a lot of character resource investment, and it's no more than a Mailman style direct damage sorcerer, or a Power Attack-based charge build, or a high-op shadow pouncer, or any other character build that invests all of its build resources in doing a whole lot of damage.

Punch away!

Answer 2

These effects won't stack in this way because there is no size above Colossal, and no damage advancement above 12d8.

The D&D 3.5 FAQ (p.113) states, as you mentioned, specifically as follows:

As a general guideline, whenever the rules don't stipulate the order of operations for special effects (such as spells or special abilities), you should apply them in the order that's most beneficial for the creature.

Lets assume that you apply in the beneficial order suggested in the question:

#1: Monk

At level 1 gains Unarmed Strike, which counts as both a manufactured and natural weapon for spells and effects which enhance either. Your party must be at least level 9 to cast earth hammer spell so you'll have 1d10 base damage or higher (2d6, 2d8 or 2d10). It deals bludgeoning damage.

#2: Greater Mighty Wallop

Increases a bludgeoning melee weapon by one size category per 4 caster levels to a maximum of Colossal. The monk's unarmed strike qualifies, and as a Medium creature it uses the chart on DMG p.28. A 16th level caster or higher can increase it up to four categories: →Large→Huge→Gargantuan→Colossal, making your 2d8 an 8d8 or your 2d10 a 12d8.

A 20th level caster can raise it five categories, but this has no benefit. The spell indicates that we use the advancement table on DM p.28, and that table has no rules for damage above 12d8, nor do the monster rules indicate a category above Colossal.

#3: Polymorph into war troll

You are now a Large creature, so your unarmed strike uses the Large Monk table: 2d8, 3d6, 3d8 or 4d8. You're still affected by greater mighty wallop, which raises the question of whether that spell adjusts when something else changes your weapon's size.

This is an ambiguous issue. Assuming that you're high enough level to have been given Colossal fists by greater mighty wallop; either your fists are now Large plus four sizes, because the spell effect is to give you four sizes higher fists, or your fists are still Medium plus four sizes, because the effect was to give you four sizes higher fists when you were still medium and the spell effect is set at casting.

However, this ambiguity is irrelevant as greater might wallop still ties you to the table, which has no rules for increasing weapon damage above 12d8, and that spell still cannot exceed Colossal, both because the spell defines that limit and because there is no such size above Colossal.

If you and your caster are below 16th level, then it may still be beneficial to cast this spell in order to push you further toward Colossal, but it cannot exceed Colossal.

#4: Earth Hammer

Your weapon damage increases by one size class as if it were one category larger.

There is no category above Colossal, nor weapon damage advancement above 12d8.

#5: Enlarge Weapon

This spell has no effect because it does not work on a weapon which has already increased in size by some other effect such as being held by a creature affected by enlarge person. A natural weapon increased in size due to polymorph fulfils this requirement.

Even if you applied this first, it would stop working as soon as you applied the polymorph.

#6: Improved Natural Attack

The progression given here only goes up to 12d8.

Note on sizes larger than Colossal: The Epic Level Handbook and Draconomicon refer to a size above Colossal, called Colossal+, for epic or truly ancient dragons. However, these rules are specific to the monster rules for advanced dragons, and Colossal+ is not a true size class, according to Draconomicon p.99:

Although there is no size category larger than Colossal, the largest advanced dragons have a greater reach and deal more damage with their attacks than other Colossal dragons. These dragons are said to be of Colossal+ ("Colossal plus") size.

According to the D&D 3.5 Monster Index, no other creature uses that size category. No general rules exist for sizes or weapon size advancement above Colossal, so even if you could gain Colossal+ fists, there are no rules in the game which allow them to deal more damage than a Colossal weapon. Nor is there such a thing as a size class above Colossal+.

While you could infer further weapon size advancement from the existing tables, any such inference would be a house rule, as the standard rules do not cover this situation.