Does the Eldritch Adept feat have an extremely limited list of invocations?

Question

The Eldritch Adept feat is presumably intended to allow a non-Warlock to dip a little into the Warlock feature by granting them one invocation. However, selection of the invocation is limited as follows:

If the invocation has a prerequisite of any kind, you can choose that invocation only if you’re a warlock who meets the prerequisite.

This line got me thinking, though. A LOT of the invocations have a prerequisite. But most of them are related to Warlock level and some are related to things which a non-Warlock could achieve with a little effort but the presence of the prerequisite is still there. To exemplify the following all have a prerequisite of the Eldritch Blast Cantrip, which most characters can get access to with a little effort:

• Agonizing Blast
• Eldritch Spear
• Grasp of Hadar
• Lance of Lethargy
• Repelling Blast

At first, I had thought that maybe this stipulation was because to use some of those invocations also required expenditure of a Warlock spell slot, which wouldn't be possible for a non-Warlock but the Thief of Five Fates invocation has no prerequisite and allows casting bane using a Warlock spell slot (which is a separate question on its own). Also, several invocations with level prerequisites simply allow the character to just cast spells without requiring use of a spell slot at all:

• Ascendant Step
• Master of Many Forms
• Otherworldly Leap
• Shroud of Shadow
• Visions of Distant Realms
• Whispers of the Grave

The list of invocations that have no prerequisite is as follows:

• Armor of Shadows
• Beast Speech
• Beguiling Influence
• Devil's Sight
• Eldritch Mind
• Eldritch Sight
• Eyes of the Runekeeper
• Fiendish Vigor
• Gaze of Two Minds
• Mask of Many Faces
• Misty Visions
• Thief of Five Fates

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With this in mind, I have two questions:

1. First, is my understanding correct that only the list of invocations that lack a prerequisite available to someone taking the Eldritch Adept feat?
2. Would it be unbalanced to permit a character to take other invocations provided they could meet the prerequisites (assume character level could be used for Warlock level)?

Answer 1

Allowing characters of any class to ignore the restriction of some Invocations being Warlock only isn't especially unbalanced, but you shouldn't do it for other reasons.

The issue isn't that any character of any class can get an Invocation and suddenly gain significant power, since most Invocations on their own aren't especially powerful (except for Agonizing Blast, as @ThomasMarkov points out in his answer, which is definitely the most powerful Invocation), the issue is that Invocations are basically the biggest portion of the Warlock class, and letting any class have access to the majority of the Warlock's class features would significantly erode the class's identity.

Imagine if you could take a feat and just pick carte blanche from any of the Fighter or Wizard or Bard class features? Everyone and their brother could be running around with Bardic Inspiration or Portent.

In my opinion, it's much less a question of balance, and more a question of keeping Warlock abilities tied to the Warlock class.

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In the comments, a comparison between Eldritch Adept and Metamagic Initiate, asserting that allowing any Metamagic options to be selected takes just as much away from Sorcerers identity as allowing any Invocation to be taken takes from Warlocks. The difference between the two features is fairly significant, however. Metamagic is, generally, fairly low impact, and can be used at most twice per day by non-sorcerers. Comparatively, even one Invocation can fundamentally alter how you play a character. Agonizing Blast, Devil Sight, and Eldritch Sight will alter how you approach combat and exploration, and having any one of these is exactly as good as if it were taken by a Warlock. The Warlock may get more Invocations, but each is exactly as powerful on a Warlock as it would be on any other class.

Answer 2

It would be unbalanced.

Your reading of the rule is correct, the rule seems entirely unambiguous.

So the question remains of balance -- and it would make this feat really good. There are some invocations that should be viewed as warlock class features that shouldn't be available to non-warlocks through a feat. BaconyRevenant gives more detailed commentary on the issue of giving non-warlocks warlock toys in this answer.

Agonizing Blast

Agonizing Blast is so good that we have a high scoring question dedicated to the problem of solving eldritch blast spam: Does Warlock combat just equal Eldritch Blast spam? Agonizing Blast is a class feature for warlocks. It makes eldritch blast so good, that there is often no reason to do anything else. This optimization question asks what the highest consistent damage output for a warlock can be, and the answer is "How many times can you cast eldritch blast today?"

We really should just view Agonizing Blast as one of the Warlock's unique class features. We even have a comparison we can make that shows how absolutely powerful it is. Consider the Evocation Wizard's Empowered Evocation feature:

Beginning at 10th level, you can add your Intelligence modifier to one damage roll of any wizard evocation spell you cast.

This is just like Agonizing Blast, except way worse because it only affects a single damage roll of a spell, and Agonizing Blast affects every hit of eldritch blast. Additionally, this is a 10th level Evocation feature, and Agonizing Blast is available at 2nd level to any warlock.

It really cannot be overstated how good Agonizing Blast is.

That said, it still takes some work for most classes to even get eldritch blast. Anyone could just take the Magic Initiate feat and get eldritch blast. Which is to say, Agonizing Blast is good enough to take two feats for -- if for some reason you don't want to just be a Warlock, but you want to play like one, and multiclassing isn't an option. Of course, if you take a 1-level dip into warlock, you can satisfy the prerequisite of Agonizing Blast and take it with the feat.

My DM already forbids 1-level hexblade dips; this just makes that option even stronger.

"At will" spellcasting is a problem.

There are a number of "Prerequisite: Nth level" invocations that give you the ability to cast a leveled spell at will without expending a spell slot. No cost, no limitations. This is just too strong to be gained by a feat. Magic Initiate lets cast a 1st level spell once per day. Taking the Visions of Distant Realms invocation would give you a 4th level spell unlimited times per day. These options are too good to be made feats available for everyone; they are class features for a Warlock.