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There is this illusory wall in a published adventure I'm running, cast by an arcane caster with CL 20. We don't have more information about the caster (e.g. their stats) or the wall (e.g. its save DC). What is the DC for the spell illusory wall? Can we determine it?

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  • \$\begingroup\$ We need to know what spell list the spell is being cast from, and we need to know the caster’s relevant ability score (e.g. Int for a wizard, Cha for a sorcerer, etc.). There is not enough information here to calculate the save DC. \$\endgroup\$
    – KRyan
    Aug 18, 2018 at 13:11
  • \$\begingroup\$ If I had all this information, I could calculate it my self. I think that there is a rule to determine the save DC just from the CL. \$\endgroup\$
    – Xxxo
    Aug 18, 2018 at 13:12
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    \$\begingroup\$ I've updated this question to reflect what we do and don't know based on your comments. I see you're sure you can calculate DC from CL alone, so I'm also adding a request to know whether we can do that at all. \$\endgroup\$ Aug 18, 2018 at 13:37

2 Answers 2

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You cannot determine the save DC; this information should have been provided to you. This is a defect in the publication.

To be thorough, the relevant rules here rely on knowing more about the spellcaster, namely what spell list they’re using and what their relevant ability score is. Specifically,

Difficulty Class

Assigning DCs is your job, but usually the rules are straightforward. The game has standard rules for the DC of a saving throw against a spell, [...]

  • Spells: 10 + spell level + caster’s ability modifier

(Dungeon Master’s Guide pg. 34)

As noted, save DC is based on spell level, plus an ability score. In core, the only place you can find illusory wall is the sor/wiz spell list, as a 4th-level spell, which means if a sorcerer casts it the DC will be 14 + Cha, and if a wizard casts it the DC will be 14 + Int. Plus any other bonuses that the character might have, e.g. from Spell Focus or whatever. But note that other spell lists could have illusory wall at a different spell level, changing the DC.

Note that caster level appears nowhere in this formula, nor can it be used to determine spell level or ability score. The relationship between caster level and spell level varies between different classes, and caster level and ability scores aren’t directly related at all—people with higher caster levels are presumably higher level, and higher-level characters usually have higher ability scores, but there is no strict mathematical relationship here.

The closest that we get to a way to determine save DC without knowing the caster’s ability scores in Dungeon Master’s Guide are the rules for

  • Magic items

    For a saving throw against a spell or spell-like effect from a magic item, the DC is 10 + the level of the spell or effect + the ability modifier of the minimum ability score needed to cast that level of spell.

    (Dungeon Master’s Guide pg. 214)

    Note that this rule actually changes how the save DCs work. Even if a character has a higher ability score than they need, items they create are still limited to a very-low save DC (which is why such items are not considered useful most of the time). Page 214 continues, saying “Staffs are an exception to the rule. Treat the saving throw as if the wielder cast the spell, including caster level and all modifiers to save DC.” The rules do not give any alternate calculation for handling staffs: you must know the wielder’s stats in order to determine its effectiveness.

  • Magic traps

    Magic traps permit a saving throw in order to avoid the effect (DC 10 + spell level × 1.5)

    (Dungeon Master’s Guide pg. 68)

    This is mathematically the same as assuming the minimum ability score for most spellcasters, but the text is worded differently for whatever reason. There weren’t any in core, but I suppose dual-ability spellcasters like archivist or favored soul might end up with a different value here?

    Anyway, I suppose you could argue illusory wall is kind of a “trap” but it’s a long, long stretch in my mind to make that statement—and are you going to allow Disable Device on it? The rules, certainly, would not have you do so, but that’s part of what it means to be a trap.

Also note that, once again, caster level is still nowhere to be found in these formulae.

Barring that, you have to guess the caster’s relevant ability score, but you would have to make some assumptions. For example, the magic item and magic trap rules assume a minimum ability score. Assuming this was cast by a sorcerer or wizard, i.e. that this is being cast as a 4th-level spell, we know that this spellcaster has at least a 14 in the relevant score, which means a DC of 16. Is this guess reasonable? In my experience, absolutely not, no spellcaster would ever have a score so low in what is absolutely their only significant concern as far as ability score goes. So DC 16 is the absolute minimum this effect could possibly have for a 4th-level spell, but it almost-certainly would not have that DC. And again, illusory wall doesn’t have to be a 4th-level spell. If some class or domain or whatever had it as a 3rd-level spell, that would change not only the DC directly for a different spell level, but also its minimum ability score, so the DC would be 14.

Going the other way, if we assume that this spellcaster has put everything they can into the relevant ability score, they would have a score of 36 at 20th level—18 base, +2 from race, +5 from levels, +6 enhancement, +5 inherent. Is it safe to assume that the spellcaster has put everything they could into this? In my experience, probably, excepting maybe the race. So a score of 34-36 is probably a reasonable guess. But it’s still a guess. Anyway, with that assumption, you have DC 26-27.

A third option, pointed out by HeyICanChan in the comments, is to assume the spell was cast by a one of the sample spellcasters found in the NPC Statistics section beginning on Dungeon Master’s Guide pg. 110. Then, for example, if the sample 20th-level wizard cast the spell with his Int 26, it would have DC 22. So far as I can tell, however, this is not the official rule suggested in the Dungeon Master’s Guide for this situation.

In practice, you should set the DC as you want, somewhere between 16 and 27, to have the difficulty you want. That will depend on your PCs. If they are similarly about 20th level, I would definitely use something in the 22-27 range. A DC 16 would be more appropriate for PCs around 6th level.

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  • \$\begingroup\$ Let us continue this discussion in chat. \$\endgroup\$
    – KRyan
    Aug 18, 2018 at 13:26
  • \$\begingroup\$ @HeyICanChan Yeah, added that, why not? It’s certainly a better idea than the minimum ability scores used elsewhere. (Though those progressions are insane; why is the wizard bumping his Dexterity?) \$\endgroup\$
    – KRyan
    Aug 18, 2018 at 14:49
  • \$\begingroup\$ (Remember that that poor wizard has NPC wealth by level so he's struggling to afford inherent bonuses anyway. Also, because he's a core wizard he believes AC is important.) \$\endgroup\$ Aug 18, 2018 at 15:04
  • \$\begingroup\$ It's more likely an author's error, unless he had some information that was inadvertently left out in publication. Nice, thorough answer, as always :) \$\endgroup\$
    – Wyrmwood
    Aug 21, 2018 at 22:18
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Caster Level is not relevant to a spell's save DC, but you can work out a minimum possible DC for a given spell

The normal formula for determining the DC of a spell effect is:

10 + Spell Level + Spellcasting Ability Modifier

For a spell of a given level X, we know that the caster must have an ability score of 10+X to cast it, as described by the ability score rules:

The minimum Intelligence score needed to cast a wizard spell is 10 + the spell’s level.

The minimum Wisdom score needed to cast a cleric, druid, paladin, or ranger spell is 10 + the spell’s level.

The minimum Charisma score needed to cast a sorcerer or bard spell is 10 + the spell’s level.

As the minimum ability score is 10+X, the minimum ability score modifier is X/2 (rounded down). So, if you know the level of a spell, you know that the minimum DC it could possibly be cast with is:

10 + Spell Level + (Spell Level / 2 [rounded down])

This is the same way that the saving throw DCs for effects produced by magic items are usually calculated:

Magic items produce spells or spell-like effects. For a saving throw against a spell or spell-like effect from a magic item, the DC is 10 + the level of the spell or effect + the ability modifier of the minimum ability score needed to cast that level of spell.

As you can see, nowhere is the effect's caster level actually relevant for determining the spell save DC. The CL is often relevant for adjudicating other parts of the spell's effect, though, like how long it lasts or how much damage it does; for magical items, the CL is by default the lowest caster level which could actually cast the spells required (you can make items with higher CLs, but the cost goes up).

In some cases, the level of a spell might be ambiguous, because in 3e many spells feature at different levels on different classes' spell lists (or might be granted as bonus spells at different levels by cleric domains and similar). If you know the caster's class, you can be reasonably sure the spell is being cast at the level it has in that class's spell list, but if you weren't sure and there were multiple possibilities, you'd have to choose the lowest potential level.

Your illusory wall.

The spell Illusory Wall only appears as a 4th level Sorc/Wiz spell, at least in the core rules. So, disregarding the possibility of other classes from extended material casting the spell at a different level, we know that the minimum possible DC is:

10 + 4 + 2 = 16

So DC 16 would be the DC of this effect if it were being produced by a magical item, and is the minimum possible DC it could have if it were being produced by an actual caster. It would be reasonable to presume that a CL20 caster probably has a better spellcasting ability score than just 14, since they must be pretty high level and otherwise they wouldn't even be able to caster their higher-level spells at all. But if you want to take that into account, you need to decide yourself how powerful the spellcaster is.

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  • \$\begingroup\$ Bravo on that edit. \$\endgroup\$
    – KRyan
    Aug 18, 2018 at 14:22
  • \$\begingroup\$ @KRyan thanks for the pointer. FWIW, you posted your answer when I was half-done composing my own and I upvoted, but I finished mine up to post anyway as the OP seemed to be taking your answer very badly and might have responded better to my different style. \$\endgroup\$
    – Carcer
    Aug 18, 2018 at 14:24

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