Can I use my best spellcasting ability for Eldritch Adept?

Question

Suppose I am a multiclass character that has "Spellcasting" from multiple sources and I take the Eldritch Adept (Tasha, p.79) feat. Can I then use my best Spell Save DC for spells that that I have gained through Invocations in this way?

For example, a Bard/Cleric that has a Spell Save DC of 12 and 15, casts "Silent Image" (PHB p.276) using the "Misty Visions" invocation (PHB p.111). Is the difficulty to investigate the image a DC 12 or 15?

I'd assume I can simply take the best option, since this seems to not be specified. Is that correct?

Now, what if my multiclass character is a Warlock/Cleric, and the Warlock's Spell Save DC is the lower one, can I still use the Cleric's Spell Save DC for spells that I cast through invocations from this feat?

Answer 1

Spell save DCs are based on the class with which a spell is associated

Generally, every spell you acquire will be a result of a feature/levelling up of a particular class. That spell will be associated with that class and will use the spellcasting modifier for that class for the DC when you cast it.

This is explained in the multiclassing rules:

Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell.

Now, Warlock invocations are a bit of an edge case perhaps. But, as Ryan C. Thompson points out in the comments, when you learn an invocation it is "from the Warlock class". Hence would use the Warlock spellcasting rules that specify Charisma to calculate any save DC.

Answer 2

(Note: This answer was written prior to the December 2021 erratum. I still believe that this is the correct interpretation for the general case of class features gained through means other than class levels, and that the erratum represents a specific exception to the general rule.)

Spells from warlock class features use your Charisma

In the class description for the warlock (as with every other spellcasting class), there is only one definition of your spellcasting ability:

Spellcasting Ability

Charisma is your spellcasting ability for your warlock spells, so you use your Charisma whenever a spell refers to your spellcasting ability.

Many Eldritch Invocations grant you the ability to cast spells without telling you which spellcasting ability to use. However, any time a warlock class feature references your spellcasting ability (even indirectly through the description of a spell granted by that feature), it must be referring to your Charisma score, because there is no other spellcasting ability it could possibly be referring to. Having levels in one or more other classes doesn't change this, as explained in the rules for multiclassing:

Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell.

The Eldritch Adept feat says (emphasis added):

[...] you learn one Eldritch Invocation option of your choice from the warlock class.

This is a bit redundant, of course, because as far as I know there are no Eldritch Invocations not associated with the warlock class, but it makes the class association explicit: you are gaining a feature specifically associated with the warlock class, which means that the relevant spellcasting ability must be the one defined in the warlock class: Charisma.

In your case, since your bard spell save DC is 12, then your warlock spell save DC is probably also 12, since they both use Charisma as their spellcasting ability. But note that you are not using your bard spell save DC; you are just using another spell save DC that happens to be the same. For example, if you attuned a Reveler’s Concertina, the spell save DC of your bard spells would become 14, but your warlock spell save DC would remain at 12. Alternatively, even if you had no bard levels at all (and hence no bard spell save DC), your warlock spell save DC would still be 12.

Answer 3

You choose one of Intelligence, Wisdom, or Charisma when you take this feat.

In December 2021, an errata to Tasha's Cauldron of Everything answered this exact question (emphasis mine):

Eldritch Adept (pg. 79). The first sentence has been re- placed with the following: “Studying occult lore, you learn one Eldritch Invocation option of your choice from the warlock class. Your spellcasting ability for the invocation is Intelligence, Wisdom, or Charisma (choose when you select this feat).”

So if your chosen invocation comes with a spell to cast, the spellcasting modifier for that spell is the ability chosen when you selected the feat, either Intelligence, Wisdom, or Charisma.

Answer 4

Use a spell save DC based on your Charisma

Whenever you cast a spell using an ability given to you by a class, you use that class's spell save DC calculation. While the Eldritch Adept feat is not itself a class feature, it grants Eldritch Invocations to characters, and those invocations are Warlock class features.

So use the spell save DC calculation given in Pact Magic for Warlocks:

Spell save DC = 8 + your proficiency bonus + your Charisma modifier

For your Bard/Cleric example character, you'd use DC 12, since your Warlock spell save DC will be based on the same attribute as your Bard spell save DC. For a character without Charisma based spellcasting, you might get a whole new DC just for spells granted to you by your new Eldritch Invocation.