Arcane Mastery: How Much Spell Penetration is Too Much?

[Doctor Despair]

Arcane Mastery is a general feat from Complete Arcane that reads:

Spoiler: You are quick and certain in your efforts to defeat the arcane defenses and spells of others.

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Prerequisite
Ability to cast arcane spells or use spelllike abilities (including invocations),

Benefit
You can take 10 on caster level checks (as if the caster level check was a skill check). You can use this feat even while under stress.

The ability to take 10 is useful when you'll succeed with an average roll, but you are worried you may fail with a poor roll. While there are feats and items one can use to specifically boost one's SR, most creatures won't have them unless the DM has a specific reason to boost their SR, as boosting SR yields diminishing returns and leads to players just using "SR: No" spells (which they'd rely on against CR-inappropriate encounters anyway). With that said, Arcane Mastery should then be a tool to let players automatically bypass the SR of CR-appropriate encounters once they have a large enough flat boost to their caster level or spell penetration relative to their ECL. How large that bonus should be will vary based on how risk-averse the player in question is, I'd imagine.

My question to you is: how large of a bonus do you think you'd need to overcome the SR of CR-appropriate encounters?

[Troacctid]

Spell resistance scales with CR. Most monsters have SR designed to provide either a 25%, 50%, or 75% failure rate against spellcasters of the same level, with 50% being the most common. This means that often, even just an extra +2 from Spell Penetration is enough. Against tougher cookies, or if you're punching above your weight class, you might need something more like +5 or +6. If you have more than that, you'll pretty much never have to worry about SR unless your DM is building weird custom monsters.

[Anthrowhale]

Arcane Mastery is also very useful for high level scrolls where the difficulty of passing the level check is highly predictable.

[Zanos]

Spell resistance scales with CR. Most monsters have SR designed to provide either a 25%, 50%, or 75% failure rate against spellcasters of the same level, with 50% being the most common. This means that often, even just an extra +2 from Spell Penetration is enough. Against tougher cookies, or if you're punching above your weight class, you might need something more like +5 or +6. If you have more than that, you'll pretty much never have to worry about SR unless your DM is building weird custom monsters.

+1. The 'highest' spell resistance you're likely to encounter that isn't just 'no' is 16+CR, which is just enough to cause a 75% chance of failure for when your caster level equals the monsters CR. With that in mind, +1 is enough to guarantee success with arcane mastery on CR = ECL opponents with 'average' SR, and +6 is necessary to guarantee success against 'high' SR opponents in that same category.

That said difficult encounters can go over CR = ECL on a single creature. Fortunately the 16+CR SR category is pretty damn rare, so you should still be comfortable with a relatively small bonus. +1 to CL from a magic tattoo is accessible, a +1 from a ring of arcane mastery, and a +1 from an orange ioun stone, WBL permitting, takes you to a +3 bonus to your caster level for everything. That's going to get you through the SR of the overwhelming majority of encounters. Less generally useful but cheaper boosts are the third eye penetrate adding a +2, and the tomebound eye of boccob, which has limited charges but adds quite large bonuses.

While you're playing with arcane mastery I'd also look into getting some bonuses to your dispel checks. Taking 10 on dispel is quite powerful since with even a small bonus you can automatically succeed on removing a huge number of spells from an equivalent caster.

[Biggus]

Spell resistance scales with CR. Most monsters have SR designed to provide either a 25%, 50%, or 75% failure rate against spellcasters of the same level, with 50% being the most common.

Where did you get these numbers from? Looking through the SRD, very few creatures seem to have them. Devils have 55%, demons 35%, angels 40% or 65%, most dragons start at 35% and decline to 25% as they age, most epic creatures 35% to 55%.

As far as I can see, most creatures have SR of between CR+8 and CR+12.

[Troacctid]

Where did you get these numbers from? Looking through the SRD, very few creatures seem to have them. Devils have 55%, demons 35%, angels 40% or 65%, most dragons start at 35% and decline to 25% as they age, most epic creatures 35% to 55%.

As far as I can see, most creatures have SR of between CR+8 and CR+12.

They often use HD rather than CR.

[Doctor Despair]

They often use HD rather than CR.

Does the average CR for a solitary creature increase approximately 1:1 with regard to average HD? All the advice thusfar seems to be predicated on the idea that most creatures you face will have approximately the same amount of HD as you have caster levels (or fewer). Is that mostly the case?

[Biggus]

Does the average CR for a solitary creature increase approximately 1:1 with regard to average HD? All the advice thusfar seems to be predicated on the idea that most creatures you face will have approximately the same amount of HD as you have caster levels (or fewer). Is that mostly the case?

Only PC class levels increase CR 1:1 with HD, with creature HD it's between 1:2 and 1:4. See here for details.

I'm a bit puzzled by Troacctid's assertion that SR is often based on HD not CR; the creatures I know of generally seem to be based on CR (all the demons in the SRD are exactly CR+8, the devils CR+12, abominations (ELH) are specifically stated to be CR+12, etc).

[Doctor Despair]

Only PC class levels increase CR 1:1 with HD, with creature HD it's between 1:2 and 1:4. See here for details.

I'm a bit puzzled by Troacctid's assertion that SR is often based on HD not CR; the creatures I know of generally seem to be based on CR (all the demons in the SRD are exactly CR+8, the devils CR+12, abominations (ELH) are specifically stated to be CR+12, etc).

A lot of templates scale by HD with regard to their SR (Phrenic is +10, Celestial is +5, etc).

A young adult black dragon's HD, CR, and SR seem to go up erratically, not really following any of these rules, varying from CR +9 to CR+6 at its youngest and oldest age categories where it has SR. Maybe the different age categories are treated as distinct creatures, and they all have different scaling? It's odd, but dragons are unique in a lot of respects. It definitely doesn't scale with HD though, as a young adult has less HD than SR, and a great wyrm has more HD than SR.

On the other hand, the terrasque has 32SR with regard to its 48HD (CR20), so that checks the "CR +12" box.

I'm thinking there may not have been a unified design philosophy (which makes sense for DnD), but a rule of thumb might be that creatures' racial SR typically scales with CR, but template SR and PC-race SR scales with HD?

Edit: Unless creatures use some fractional scaling with their HD to SR conversion... that would complicate things.

[Biggus]

A lot of templates scale by HD with regard to their SR (Phrenic is +10, Celestial is +5, etc).

Ah OK templates, gotcha.

A young adult black dragon's HD, CR, and SR seem to go up erratically, not really following any of these rules, varying from CR +9 to CR+6 at its youngest and oldest age categories where it has SR. Maybe the different age categories are treated as distinct creatures, and they all have different scaling? It's odd, but dragons are unique in a lot of respects. It definitely doesn't scale with HD though, as a young adult has less HD than SR, and a great wyrm has more HD than SR.

Looking at the ten types of true dragons in the MM, while their SR is slightly erratic, it pretty closely follows CR. For example, 7 out of 10 have SR of exactly CR+8 at young adult (the others have +6 or +7) and 9 out of 10 have exactly CR+6 at great wyrm (the other has +7).

I'm thinking there may not have been a unified design philosophy (which makes sense for DnD)

QFT.

but a rule of thumb might be that creatures' racial SR typically scales with SR, but template SR and PC-race SR scales with HD?

That makes sense, as PC/humanoid races are expected to increase their CR by taking class levels, which do scale 1:1, at least in theory.

Looking at the celestial and fiendish creature templates, I see their SR is both capped at 25, which makes sense as otherwise you could eg make a fiendish tarrasque with SR of 53 but a CR of 22.

[Yogibear41]

Most Outsiders have HD pretty close to their CR in my experience.

Spell Penetration + Arcane Mastery, will probably let you handle 90% of same CR creatures.


Beware the Spellgaunt from MM2, man screw those guys.

[Zanos]

I wouldn't bother taking spell penetration as a feat, since there's some decent items that are relatively cheap by the time you are regularly encountering foes with spell resistance.

[Kaleph]

I typically rely on 3rd eye penetrate and orange prism ioun stone, plus assay spell resistance (prepared once) for the very though enemies, plus a certain amount of SR:no spells for those straight immune to magic.

That's a +3 boost vs SR (which somewhat matches with the rules-of-thumb that the others are suggesting), it kinda works based on my experience, and it doesn't burn a feat.

[Thurbane]

Arcane Mastery is also very useful for high level scrolls where the difficulty of passing the level check is highly predictable.

Indeed - there are many CL checks not related to SR where the feat can be useful.

Break Enchantment spell
Overcoming a Nondetection effect
Scry Location spell
Steal Summoning spell
Suppress Magic spell

etc.

[Troacctid]

I tend to like it on escalation mages.

[Doctor Despair]

It seems like the tenative conclusion of the thread is that you'd ideally have at least caster level +2 when using Arcane Mastery, but that you may want to purchase the following items for a cumulative +5 (or +9-13 for limited times each day) depending on whether you go against enemies with strong SR (CR +6) or CR-inappropriate encounters:

Spoiler: CL-Boosting Items

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Ring of Arcane Might (+1 caster level, 20,000gp)
Orange Ioun Stone (+1 caster level, 30,000gp)
Scrolls of Create Magic Tattoo CL13 (+1 caster level, 750gp/scroll, lasts 24 hours)
Third Eye Penetrate (+2 to Penetration, 8,000gp)
Tomebound Eye of Boccob (+4/6/8 to Penetration, 7,000GP (3 charges/day))

In addition, I think the following feats are a somewhat complete list of feats that increase caster level or spell penetration:

Spoiler: CL-Boosting Feats

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True Caster-Level Boost
Practiced Spellcaster (+4 caster level, max HD)
Primitive Caster (+1 caster level/component added)
Dark Speech (+1 in exchange for 1d4 cha damage)
Reserves of Strength (+1/2/3 caster level, stunned for 1/2/3 rounds (or 1/3/5d6 damage))
Elder Giant Mage (Add 1-3 rounds to casting time; +1 CL for each round added)
Divine Spell Power (Bonus to CL scaling with turn result)
Metamagic Vigor (+1 to second spell cast affected by the same metamagic)

Situational Caster-Level Boost
Cormanthyran Moon Magic (+2 when in moonlight)
Earth Spell (+(1+X) to caster level, where X is the number of slots spell is heightened; only on stone/unworked earth)
Ship's Mage (+1 to caster level while on a ship you've lived/worked on for 1 week)
Storm Magic (+1 while in a storm)
Lunar Magic (+1 or -1 depending on the phase of the moon)
Ceremonial Empowerment (+1 on your god's holy days)

Boost to Caster-Level for Specific Purposes
Demon Mastery (+1 when summoning a demon)
Infernal Sorcerer Heritage (sorcerers gain +2 to conjuration to summon evil outsider)
Hin Wandermage (+1 to travel spells; race-specific)
Shield Dwarf Warder (+1 caster level enhancing or creating armor/shield, race-specific)
Gold Dwarf Dweomersmith (+1 when casting spell to create or enhance weapon; race-specific)

Boost to Caster-Level Checks
Grell Alchemy (+2 to penetration; +2 to dispel DC)
Spell Penetration (+2 to penetration)
Greater Spell Penetration (+2 to penetration)
Fortify Spell (increase spell's level by X; increase penetration by 2/level increase)
Soultouched Spellshaping (+Essentia to penetration and dispels, grants 1)
Soultouched Spellcasting (Same as above)
Spell Rehearsal (+2 to penetration on the second consecutive time you cast the same spell on the same target)
Pious Spellsurge (+1d6 to caster level for 1 spell, costs 2 faith points)
Heroic Destiny (+1d6 to CL check; race-specific)

Boost to Caster-Level for Specific Schools
Reserve Feat (+1 to specific school)
Shadow Weave Magic (+1 to DC/penetration for enchantment, illusion, necromancy; -1 caster evocation, transmutation)
Abyss-Bound Soul (+1 to illusion, or enchantment, or necromancy)
Aerenal Half-Life (+1 CL for necromancy)
Fae Power (+1 to CL/DC for enchantments)
Mutable Body (+1 to transumation spells cast on you)
Greater Path Focus (+1 CL/DC from chosen path's mysteries)
Path Focus (+1 to DC/CL on chosen path's mysteries)
Domain Focus (+1 CL to spells from domain spell slots)
Tattoo Focus (+1 to specialized school, +1 to penetration for same school, race-specific)
Enchanting Song (+1 to CL/DC for enchantments; costs bardic music)
Misleading Song (+1 to DC/CL of an illusion spell; costs bardic music)
Infernal Bargainer (+2 to specific divination spells; outsider-specific)
Hellsworn (+1 to evil spells)
Fiendish Power (+1 to CL/DC of evil spells)

Boost to Caster-Level Checks for Specific Schools
Shadow Song (bards get +1DC, +1 penetration for Enchantment and sonic spells)
Svirfneblin Figment Master (+1 DC/penetration for illusion (figments); race-specific)

Boost to Caster-Level for Specific Spells
Spell Thematics (+1 for 1 spell/spell level)
Words of Draconic Power (+1 to language-dependent spells)
Arcane Thesis (+2 cl for one spell)
Corrupt Arcane Studies (+1 DC/Penetration for Sorc/Wizard spells)
Elven Spell Lore (+2 to CL using dispel magic/greater dispel magic)
Chondathan Missionary (+1 caster level for specific spells, or +2 if you are proselytizing)
Jester's Magic (+1 for specific spells)
Portal Sensitive (+2 to caster level checks on analyze poral, race-specific)
Eldritch Linguist (+1 using specific spells; race-specific)

Boost to Caster-Level for Specific Descriptors
Frozen Magic (+1 to cold spells, or +2 to cold when in areas of extreme cold)
Cali$hite Elementalist: Fire (+1 caster level for air traditions or fire traditions)
Bloodline of Fire (+2 caster level for fire descriptor)
Elemental Spellcasting (+1 using chosen descriptor)
Draconic Power (+1 CL, +1DC to specific descriptor or subschool)

Situational Boost to Caster-Level Checks
Sacred Tattoo (+1DC/penetration while in a consecreated/desecrated/hallow/unhallowed area to your deity)

Boost to Caster-Level Checks for Specific Purposes
Pernicious Magic (+4 to penetration against weave-users)
Overcome Shadow Weave (+5 to detect, counterspell, dispell Shadow Weave effect)
Oni's Bane (+3 to penetration against outsiders)
Dragonfoe (+2 penetration against dragons)
Favored Magic Foe (+1 to penetration against favored enemy)

Part of what prompted me into this line of questioning was for general consideration in caster builds, but also with regard to a specific build (referenced here).

The build loses a single caster level at level 7, so it needs an effective +3 between two feats to effectively break SR (Grell Alchemy + either Practiced Spellcaster or Sickening Grasp), or a +2 to dispel (Practiced Spellcaster and Spell Thematics is probably best?). What do you all think? If you're down one caster level, which two feats do you think would best help you catch up in the SR/Dispel Magic race to stay on-CR?

[Zancloufer]

One interesting thing to note about some bosses (especially dragons) is that with how D&D encounters happen in RAW (let alone actual modules, or custom enemies modded/made by DMs) is that fighting a CR +4-6 enemy is not unheard of. Combine that with the CR +6-12 SR you would expect one would expect a SR of about ECL +18 to not be unreasonable in something you might encounter. Mind you most enemies with a CR of ~6+ or more are probably things not meant to have a straight fight against, but still quite possible to encounter.

As such in the case where you can safely take 10 I would say that a spell penetration bonus of +8 would be towards the top of practical optimization, with +17 Spell pen/CL boost if you don't have a way of reliably taking 10.

[Doctor Despair]

One interesting thing to note about some bosses (especially dragons) is that with how D&D encounters happen in RAW (let alone actual modules, or custom enemies modded/made by DMs) is that fighting a CR +4-6 enemy is not unheard of. Combine that with the CR +6-12 SR you would expect one would expect a SR of about ECL +18 to not be unreasonable in something you might encounter. Mind you most enemies with a CR of ~6+ or more are probably things not meant to have a straight fight against, but still quite possible to encounter.

As such in the case where you can safely take 10 I would say that a spell penetration bonus of +8 would be towards the top of practical optimization, with +17 Spell pen/CL boost if you don't have a way of reliably taking 10.

To be fair, if you're in a CR+6 encounter against an enemy with SR, it's probably a better use of resources to just restrict yourself to SR: No spells, isn't it?

[Anthrowhale]

If you are interested in dragon magazine, then Theurgic Specialist (Dragon #325) and Psiotheurgist (Dragon #349) can be quite potent. Of course, they also require some setup in class choices as well.

[Zancloufer]

To be fair, if you're in a CR+6 encounter against an enemy with SR, it's probably a better use of resources to just restrict yourself to SR: No spells, isn't it?

Depends. A good example is fighting an almost "impossible" CR true dragon. On paper SR:No spells might be better, but if you can afford to overcome their SR something like Empowered (preferably though a rod) Shiver Touch is essentially a OHKO against almost all dragons. The problem is that many "I Win" buttons are SR:Yes.

[ciopo]

list of CL boosts by categories

I think there was a "flaw" that is +1CL to specific school, -1CL to all other schools, not a flaw, but it was a *thing* you could take only at first level, I forgot what it is but I'm fairly sure it's 1st party

[Anthrowhale]

I think there was a "flaw" that is +1CL to specific school, -1CL to all other schools, not a flaw, but it was a *thing* you could take only at first level, I forgot what it is but I'm fairly sure it's 1st party

I believe you are referring to the trait Spellgifted.

[noob]

I believe there is a template that gives sr based on hd.
In which case you would need arbitrary amounts of spell piercing against creatures with heaps of hd.

[Doctor Despair]

I believe there is a template that gives sr based on hd.
In which case you would need arbitrary amounts of spell piercing against creatures with heaps of hd.

Eventually, I suppose it would be most efficient to just call it a loss and go for SR: No, but up to a point, I suppose Fortify Spell would be the right call, eh? Or go into Dweomerkeeper...