Is there anything preventing warlocks from learning and casting invocations listed in warlock prestige classes?

Question

This question has opened a Pandora's Box in my head. The first answer (and resultant discussion) states that warlocks are unable to learn invocations described in prestige class descriptive text because they are not on the warlock invocation list and should be treated as class features. However, if you look at the Eldritch Disciple prestige class (CM p.53) you will see where there is a class feature designed that takes the place of an eldritch essence without being an eldritch essence (see Gift of the Divine Patron (Su) - Healing Blast on p.54).

Then there is the Eldritch Theurge prestige class (CM p.57). It specifically states in the descriptive text that "All eldritch theurges have levels in the warlock class" since only warlocks level both invocations and have eldritch blast. Additionally, it does not grant the ability to cast any new types of invocations:

Invocations: At each level, you gain new invocations known and an increase in caster level as if you had also gained a level in an invocation-using class to which you belonged before adding the prestige class level. (This includes eldritch blast.) You do not, however, gain any other benefit a character of that class would have gained. If you had more than one invocation-using class before becoming an eldritch theurge, you must decide to which class to add each level for the purpose of determining caster level and invocations known.

At levels 3 and 10, it grants bonus invocations in addition to the invocations that would be earned through invocation-class progression. Those are Spellblast, defined as a Lesser invocation, 4th-level equivalent, Eldritch Essence, and Greatreach Blast, defined as a Greater invocation, 5th-level equivalent Eldritch Essence.

There are additional invocations defined and granted for the Enlightened Spirit (CM p.61).

Also note that all of these invocations are listed by WoTC on their official Invocation Index

The summary and root of the question is this: Is a warlock not able to learn these invocations despite the fact that they are designed as invocations with lesser/greater/etc. status instead of class abilities (as the same book did with the healing essence-like class ability)? Is there any actual rule stating that these invocations are not, in fact, usable by warlocks? It appears to me that much like feats that are defined in a prestige class description, any warlock who is able to learn least/lesser/greater invocations, as applicable, should be able to learn these. It's just that members of these prestige classes additionally get the invocations as bonus invocations instead of through standard warlock invocation progression. Otherwise, if these features were to be granted only as class features, why would they be designated as least/lesser/greater?

Answer 1

The default for any invoker class is to be unable to choose any invocation except those explicitly assigned to it. Complete Arcane, Complete Mage, and Dragon Magic all explicitly assign various invocations to the warlock class. Complete Mage does not, however, include greatreach blast or spellblast in the list of invocations it assigns to the warlock, nor does it assign those invocations to the warlock separately when they are described within prestige class abilities. As such, those invocations are not available to a warlock unless or until he gains the class feature that offers them.

To see this, let’s start with the very definition of warlock invocations:

Invocations: [...] A list of available invocations can be found following this class description, and a complete description of each invocation can be found in Chapter 4 of this book.

^{(Complete Arcane pg. 7)}

The warlock can literally learn only those invocations that are listed in its description there. At no point in the entire column-and-change description of the invocations ability does the warlock gain any ability to gain any other invocation; the class feature only enables the selection of those invocations listed within the class description itself.

That doesn’t mean that these are the only invocations a warlock can ever take, however! D&D 3.5e is an exception-based ruleset, which means future material can, and does, make exceptions to earlier rules. That isn’t the point. The point is that the default state for any given invocation is for it to be not an option for a warlock. Something has to make it an option for a warlock in order for it to become one.

And the books consistently do exactly that when introducing new invocations. For example,

Also included are over twenty new invocations for the warlock class found in the Complete Arcane supplement. These invocations are governed by the rules in that book.

^{(Complete Mage pg. 89)}

That quote is from the beginning of Chapter 4, Spells and Invocations, of Complete Mage. It specifies that these new invocations are for the warlock class. That provides an exception to the rule on page 7 of Complete Arcane that says a warlock’s options are only those invocations included right there on pages 8 through 10 of Complete Arcane.

Not listed in Chapter 4, however, are spellblast or greatreach blast. Those invocations are described only in the descriptions of the relevant prestige classes—and those descriptions only give them to those prestige classes, rather than make them available to just any warlock. (In fact, the word “warlock” does not appear in the description of either class feature.)

For another example, consider Dragon Magic, which presents the dragonfire adept class, another invoker. These quotes are the references to its invocations:

Invocations: [...] See Draconic Invocations, below, for a list of available invocations.

^{(Dragon Magic pg. 25)}

Draconic Invocations

Each draconic invocation falls into one of four grades: least, lesser, greater, or dark. These invocations are described briefly below. See Chapter 3: Draconic Magic for full descriptions.

^{(Dragon Magic pg. 26)}

The invocations available to dragonfire adepts (see page 24) are described in this section. The format for invocation descriptions is given below, followed by the lists of invocations available to dragonfire adepts.

^{(Dragon Magic pg. 78)}

Note that none of these make any mention of the warlock class, and yet they are presenting “invocations,” and specifically, invocations graded into four tiers of least, lesser, greater, and dark. The invocation format does not include an associated class, as the spell format does, either, and the dragonfire adept reprints several invocations that Complete Arcane and Complete Mage had made available to the warlock, under this heading that says they’re available to dragonfire adepts as well.

And as perhaps the final nail in the coffin, Dragon Magic also presents several new warlock invocations. There are in an entirely different section, which reads

Because the powers of warlocks (see Complete Arcane) are tied to their dark nature, they're usually incompatible with draconic magic. However, warlocks living in a world where dragons are more active learn to tune their powers toward the destruction of such foes. These new invocations are intended for use by warlocks who expect to face dragons (or similarly formidable spellcasting opponents) in combat.

^{(Dragon Magic pg. 81)}

Note again the complete lack of anything here saying that dragonfire adepts cannot take these new warlock invocations; per Dragon Magic pg. 25, there is no reason to do so because these invocations do not appear in the list that begins on page 25, so they are unavailable to dragonfire adepts by default.

We have an existing Q&A about dragonfire adepts and warlocks and whether they can take each other’s invocations—and it comes to the same conclusion as this answer, that invocations are class-specific and can only be taken by a dragonfire adept or warlock if they are printed in a section assigning the invocations there to that class.

So yes, there is something preventing regular warlocks from taking those invocations: those invocations are simply never made available to them unless or until they gain the class feature from the relevant prestige class that makes them available.

Answer 2

From Complete Arcane, page 8:

Some of a warlock’s invocations, such as frightful blast, modify the damage or other effects of the warlock’s eldritch blast. These are called eldritch essence invocations. [emphasis added]

This defines an eldritch essence specifically as a warlock invocation. There is no other definition of eldritch essence in the entirety of 3.5e D&D.

From Complete Mage, page 54:

Healing Blast: In place of applying an eldritch essence invocation to your eldritch blast, you can change it into a healing blast.

This establishes that the authors are capable and do construct class abilities that functions similarly to an eldritch essence but are not defined as such.

From Complete Mage, page 57:

Spellblast (Sp): This invocation (Lesser; 4th; Eldritch Essence), ...

Since it has been established that the authors are perfectly capable of assigning a class ability that functions similarly to an eldritch essence but is not defined specifically as one, it is safe to assume that these sections do define an eldritch essence specifically, which is defined in Complete Arcane as a invocation used by warlocks.

Additionally, the language in Complete Mage on page 89 states:

Also included are over twenty new invocations for the warlock class found in the Complete Arcane supplement. These invocations are governed by the rules in that book.

First, the language of this statement defines an inclusive list not exclusive, meaning that the items in this chapter are being added to the warlock invocation list, but there is no language implying that other items are not also allowed on the warlock invocation list.

Second, and perhaps more importantly, the language explicitly states that the invocations are governed by the rules stated in Complete Arcane, which established already that eldritch essence invocations are warlock invocations used to modify eldritch blast.

Therefore, warlocks would be able to learn any invocation defined as a least/lesser/greater/dark eldritch essence invocation since all of the rules have established what an eldritch essence is, and that no rules have stated that certain items bearing that designation somehow break the rules.