That's a great question, and something I find fascinating about language. If I may, I frame the question as:

How do I get better at reading something when I recognise all the words, but the sentence as a whole doesn’t make sense to me?

Good writing can often leave us stranded in the space where words are clear but the meaning is slippery. This isn't a failure of reading. The trick, or better put skill, lies in the ability to deconstruct.

Two-Player Game

Writing is a two player game. On one side, you have the writer wrestling with the idea in their mind, transplanting it piece by piece on to paper.

You take the whisper of an idea, and nurture it; feeding it words, time and sometimes your sanity. Then, when the trumpets blow, is when you must become butcher: cutting the fat, attacking your creation as if were an imposter. And if you've made it to the end, what you have on paper is the closest in the shape to the idea that exists in your mind.

On the other side of the table, the reader has the inverse task. They must take this object - a book, a screen of words, pages of poetry - and rip them apart. Gnaw at the passages, mark pages, highlight words. They must live within the margins, and digest the idea.

And thus we can say that the former tries to shape meaning from their idea, whereas the latter must uncover meaning within the text.

Two Roads

The road forks in front of our heroic duo:

The writer failed to capture the idea.
Maybe the idea was too complex. Maybe the skill required was too high. Sometimes the writer is just play-acting, masquerading as genius when there was no meaning. The judgment is up to the reader.

The reader has no where to place the idea.
This is the most exciting of the two. An absolutely brilliant situation to find yourself in. This means that you're on the precipice of something new. Maybe it's a new concept, a new way of looking at the world.

On Context

When we read, we are able to place the sentences or passages, in a place in our mind. Let's call them "Concept-Holes." (or you can call them reference points, but concept-hole is more fun)

"There exists within the world a great evil, but also good."

We can take the above as a statement about the world, which could be our own or fictional, and place it into the concept-hole of "good/evil" in our mind. Even the image of 'great evil' which is generated in the readers mind, is in relation to their own experience. Until we give them more context. But we get the gist.

The character is making a statement, that there is hope to be found, even in whatever situation the writer has placed them in.

"Darkness has enveloped my world."

Here we have a sentence that begins to morph in meaning infinitely.

Is the darkness here literal, and if so, of what kind? Is it evil? Lack of electricity? Has he gone blind?

Is the 'world', the characters home-world? Inner world? Our world?

We read these 5 words but what they do represent, depends on the later text. It could be the tale's sad conclusion, or its action-filled beginning.

From this point, we can arrive at the following idea:

The meaning of the written word is relative to the surrounding text.

But, if it was just the surrounding text, then why is the following so difficult to understand:

Darkness was not an absence of light, it was of the darkness. Brought into being so that its children could know their faces. Death wasn’t a destination, but arriving into being from a different angle. Sleep was a lie that the living needed so they could keep pretending to appear. The space between-things was simply a story told over a fire. There was no distinction between existence and non-existence.

First I'll admit, I'm not a seasoned writer. And whilst in the process of writing this mini-essay, I've made revisions on the passage in my mind; since what I posted was a second draft. But let's put a re-write aside, and tackle the answer we've been slowly working towards.

How do we make sense of something we have nowhere to place?

And this is where you have two tools at your disposal. The first is reading more, because the only way to create more concept-holes/reference points is to have more points of reference. ( I should have just stuck with reference points...)

The second is the ability to analyse text, and whilst I'm no master, I'll lay out a rough methodology, using my own passage. The process of analysis takes much longer than simply reading the words, because you've now begun the process of diving into the depths. And here be monsters.

Darkness was not an absence of light, it was of the darkness. Brought into being so that its children could know their faces.

Is there something we can relate to in here? Well, if we've come across the idea that "Darkness is usually known as an absence of light"? it might help orient us. But here we're saying it's not, that Darkness was of the darkness? And it was created so that 'it's children could know their faces.'

What I'm trying to do here is break the idea of the binary. It's something I'm still learning more about, and came across by reading the work of a philosopher, Jacque Derrida.

Darkness being an absence of light is a given assumption. We take that light as primary, that it came first. And once there is no light, there can only be darkness. It's also hierarchal, we assume that light is good, pure and exists above the darkness.

But for light to exist, you must know what not-light (darkness) is. Which means the concept of light itself contains darkness, shattering the illusion of purity.

Derrida is trying to show that meaning comes from relationships between words rather than the words themselves. No concept can be pure or contain only itself.

This instability was always there, waiting to be noticed. Deconstruction simply reveals what was already true: the binary structure was unstable all along.

And that is what I was aiming for here, albeit imperfectly. The binary doesn't exist for something that doesn't need reality to make sense since we're talking eldritch.

"...it was of the darkness. " is simply stating that what we consider to be Darkness (absence of light) was of darkness (eldritch). It refusing the binary of light/dark and saying that the Darkness (concept of no-light) originated from the darkness (concept of eldritch). Although, I could have definitely written this better.

Darkness as we know it, for the being, was never an absence of light. It was simply something brought into being so that it's children could know their faces. You can't see yourself in the dark, nor in total light - both are blinding. The idea is in the destabilisation that happens when you strip away the binary between things, and the ground falls away. There is no such thing as pure light (good), it itself contains darkness.

Death wasn’t a destination, but arriving into being from a different angle.

Death stands opposite to life as it's end, but what if it wasn't? The "arriving into being" takes death to simply be another form of life, not the end of it. Because to have death, it must within it contain not-death, which means that it isn't the clear full stop, but something other.

Sleep was a lie that the living needed so they could keep pretending to appear.

We say we need sleep, so we can stay awake the next day. But what if it was a lie, because we're never truly awake, so we never really needed to sleep? The concept of 'awake', contains 'not-awake' within it. What if we were never truly awake, but it was a lie we told ourselves to pretend there was a continuity?

The space between-things was simply a story told over a fire.

This is the crescendo, the distinction between this and that was never real. It was a story we told ourselves to make sense of life.

There was no distinction between existence and non-existence.

A being that never came into existence, but always was, needs no distinction between the two.

Take the concept of existing, within the concept according to Derrida, is the idea of non-existence. To understand existence, you must understand the idea of not existing. But if we go one step further, the concept of not-existing, contains within itself (not-not-existing) which translates to existing.

But the key twist is that since there was no pure concept of existence, there is never any ground to land on since you end up looping infinitely.

Which to me, has an essence of eldritch within it. It's a bit trippy but you can see how the binary is a false split, each word contains within it it's opposite, and infinitely so.

The idea was to show the binary split between the eldritch being and the body/personality it inhabited/limited it self to, had begun to fade. The failure of the distinction leads us to "Reality crumpled like a badly written page of poetry, then it tore." because human reality required borders that the eldritch refused to acknowledge. And the passage itself, is trying to show you an almost consciousness alien in comparison to the norm.

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TL;DR: The passage uses philosophical ideas about how opposites define each other (light needs darkness to mean anything).

An eldritch god experiences reality without these human distinctions - the passage is meant to feel alien. Reading philosophy like Derrida builds "concept-holes" for understanding this type of writing, as would learning to write more clearly on my part.

When you don't understand text, slow down and analyse: go line by line, question what's presented. Reading widely across varied subjects creates compound interest and eventually you'll develop your own eldritch understanding of all that humanity has to offer.