When stacking Energy Substitution and Energy Admixture of the same energy type, can Mastery of Elements be used to maintain different energy types?

Question

In Dungeons & Dragons 3.5e, using the Archmage prestige class from the Dungeon Master's Guide (DMG), I've chosen the Arcane Thesis feat to allow the use of metamagic feats with a +0 level adjustment, reducing the spell's level adjustment cost incurred by more expensive metamagic feats.

So, if I'm using the +0 metamagic feat Energy Substitution (cold) on the thesis spell, and that's the only energy substitution feat I've included in my build, applying it to lower the adjusted level cost of also applying Elemental Admixture, could I then use the High Arcana (Mastery of Elements) to prevent the damage type from negating the bonus damage from Elemental Admixture?

I believe it should work, but I can see room for argument, so I'm seeking some opinions. Here's my reasoning:

Metamagic feats are applied during preparation. Therefore, a prepared spell with both metamagic feats applied would qualify as a cold spell when prepared. Mastery of Elements explicitly states that you choose the alternate energy type at the time of casting, which is after the preparation phase. Energy Admixture provides the altered damage separately from the spell's prepared damage type. Without Mastery, the two would cancel each other out, allowing only one damage type to reach the damage cap. However, at the time of casting, Mastery can select to change the prepared type again from the metamagic-altered cold type to, let's say, Sonic. The unaffected cold damage from Energy Admixture would then function as usual.

This situation is slightly complicated by the fact that I accessed the Archmage class from the Sorcerer class instead of the Wizard class. However, I believe the logic still applies. While spontaneously cast spells are not technically prepared spells, they do take more time to cast, indicating a 'preparation step' prior to beginning casting, allowing room for effects like Energy Substitution and Mastery of Elements.

I look forward to hearing people's thoughts. Please do let me know if I've omitted any pertinent details. | | | | To add a little context. This build is a Sorcerer (5) Incantatrix (10) Archmage (4) with a variety of complicated build rules adjusting things all over the place.

The Thesis spell in question is Meteor Swarm (the 9th level sorcerer/wizard spell), and the list of metamagic effects I'm employing is extensive. Arcane Thesis is used to keep the spell slot at 9 in conjunction with the Incantatrix 10th level class ability Improved Metamagic (PLayers Guide to Faerun Pg63, and several instances of the feat Practical Metamagic from Races of the Dragon Pg101.

The way the interactions work I essentially need 1 +0 metamagic feat for each +1-+4 metamagic feat applied to allow Thesis to bring the spell level adjustment back down to +0. This methodology is incredibly feat intensive so knowing if my Class ability can offset my list of needed +0 Metamagic feats is critical to ensuring the build is valid.

Answer 1

You can, but you don’t need to.

Neither Energy Substitution nor Energy Admixture negates any damage; it’s not clear what you mean when you worry about “negating the bonus damage from Energy Admixture,” since nothing here should negate that bonus damage under any circumstances.

Note that Energy Admixture can explicitly be combined with Energy Substitution to simply double damage, without creating a “mix”:

Energy Admixture

[…]

Special: […] You can use Energy Admixture to further alter a spell that has already been modified with Energy Substitution, and you can also admix your chosen energy type with a spell that already uses the same type, in effect doubling its normal damage dice.

^{(Complete Arcane, pg. 78)}

This says you can use Energy Substitution (cold) on fireball, and then use Energy Admixture (cold) on the same cold-substituted fireball, allowing you to do 2d6 cold damage per caster level (to a maximum of 20d6 cold damage).

Now, you can use mastery of elements to change the cold-substituted cold-admixed fireball into another type of elemental damage, say, acid, so it deals 2d6 acid damage per caster level (max 20d6 acid damage). That is nice because it saves you from having to take Energy Substitution (acid) and Energy Admixture (acid), which is very costly. But you don’t have to to get your full damage; you would only do this when you want acid damage instead of cold damage for whatever reason (targets’ resistances and/or vulnerabilities, presumably).

By the way, it’s a good thing that Energy Admixture explicitly spells out how the two effects combine, because otherwise attempting to “logic” our way to how it works is very fraught. The ordering and combining of metamagic feats is not really super-clearly spelled out by the rules. The ordering of effects is also not well-defined by the rules. You are certainly correct that preparation and casting are different moments in time, so for the wizard there is a clear separation where first one thing happens, but your claim that “it takes more time to cast, indicating a ‘preparation step’ prior to beginning casting” doesn’t really fly, under the rules. The rules say nothing about that; adding metamagic to a spell, for a sorcerer, happens during casting, just the same as mastery of elements, and the game does not recognize any subdivisions or distinctions within that action. The FAQ suggests—as a rule of thumb—that the controller of simultaneous effects should get to choose their order, so the sorcerer could—under this guidance—choose to order these things the same as the wizard, but the rules don’t actually enforce that and it is up to the DM to decide that’s acceptable in this case.