Reports mention witnessing entire sections that had collapsed return to their original position, only to be shattered again in an instant.

This is a very cool line.

Foreing contamination

Did you mean "Foreign"?

These objects usually have unique properties and have only been found in this level.

I don't think it makes sense to say they are only on this level, since the Voices of the Dead were written beforehand and presumably aren't specific to this level.

Returning only static in the case of communication devices and inconsistent signage in the case of gadgets built for the purpose of facilitating navigation through the Backrooms.

I think this is an incomplete sentence. Shouldn't be separate from the previous sentence. Also, put a comma after "devices".

more and more scarce over the time

over time*

The last attempts to form a bastion against the siege of the unknown.

Also an incomplete sentence that shouldn't be separate from the previous.

Tightly intertwined with The Halls are the Crumbling Dungeons, a labyrinthine complex reminiscent of a medieval castle's undercroft. The level presents a high level of decay. Cracks and debris are a common sight, with entire areas on the verge of collapse, waiting for the slightest touch to succumb. Every few rooms, torches can be found hanging on the walls, but they have long since been extinguished, leaving the level in complete darkness.

You have hooked me already! I have waited for quite some time for this kind of limspace and from this paragraph alone, I can tell it will be great! Though I don’t think we use “levels” anymore. It could work but might need to be replaced with something like “space” or “limspace”.

The physical structure of the level is not the only element in decay, as the Crumbling Dungeons have a significantly reduced dimensional stability. Throughout the level, one can observe areas where the very fabric that constitutes the Backrooms is weakened. Reports mention witnessing entire sections that had collapsed return to their original position, only to be shattered again in an instant. This erratic behavior is what provides most of the level's danger, as one can suddenly find themselves trapped inside a newly regenerated room, hit by a fragment of a wall that was shattered for no apparent reason, or, in the case of the unluckiest, buried inside a wall that reappeared without warning.

Great so far. The last sentence is a bit long but conveys its point.

Foreing contamination
In certain places, the characteristic wallpaper of The Halls can be seen intersecting with the ancient brick walls of the Crumbling Dungeons. These sections of wallpaper can be seen "growing", consuming the Crumbling Dungeons' walls and transforming them into what appears to be a new segment of the The Halls. This "growth" is, like the vast majority of the level's properties, unpredictable, which means that a recently transformed area may take years to advance a few more meters or cover dozens of chambers in seconds.

“Foreing containment” may be a misspelling of “foreign containment”
“the The Halls” is a typo? It would work better just as “The Halls”
I love the wallpaper growing like fungus or moss. Crazy visuals!

Obscure frontiers
The further you move away from the entry points of the level, the more prevalent its "moribund" state becomes. If you go deep enough into its ruinous corridors, you may encounter the so-called "limit" of the Crumbling Dungeons. This is an area where the structural sequence of the Crumbling Dungeons is completely cut off, giving way to what appears to be an infinitely stretching void. This void seems to completely surround the level, as it can sometimes be observed in other areas of the Crumbling Dungeons.

Why is “moribund” in quotes? That is exactly how the space is. Places can have traits like living creatures.
This void sounds interesting. Develop it later.

Some have claimed to have caught glimpses of the room clusters that conform various other levels suspended in the distance of this same void. The veracity or significance of these claims has yet to be determined. The fate of those who fall into this void can only be speculated, which gave free rein to all kinds of stories and legends. From endless falls to the most gruesome deaths and tortures. What is certain is that no one who has fallen through one of these rifts has ever returned to tell their story.

“Conform various” to “conform to various” or “come from various”?
A while ago, I had written a void aspect of a level and I fumbled it. Your void description is phenomenal! I love stories and legends.

Distinctive relics
Remains of extremely deteriorated objects, to the point of being unrecognizable, can be found on a regular basis. Occasionally, the circumstances in which the objects were abandoned has allowed their partial preservation, which has facilitated the recovery of a number of items from within the Crumbling Dungeons over time. These objects usually have unique properties, and have only been found in this level.

“Has” to “have”
Remove comma after “properties”?

A small barrel full of a liquid with a slight scent of almonds.

We removed all mentions of almond water last week. Could either replace it or have some other mystery.

A Voice of the Dead with a crack at the top of the skull. No matter how long it was held, the Warning skull emitted only unintelligible mumbles.

Yay A Voice of the Dead! I wrote them. Anyways, interesting storytelling with it. That is their purpose after all! Something so dangerous/ unknown that the voice can’t clearly explain the threat.

A small sealed wooden box. No one has been able to open it or damage it in the slightest. Each person who holds it perceives something different within it through the box's weight, smell and the sound it makes when shaken.

Add a comma after “smell”
Interesting object… the mystery writes itself here.

Three vials that appeared to contain diluted Amartía fruit juice. Upon closer inspection, however, it was discovered that unlike its usual color, this juice had a milky white coloration. Individuals who agreed to drink the liquid, in exchange for a hefty reward, described experiencing a thirst that could not be quenched in any way. This effect lasted for about a week.

Add comma before “unlike”
Interesting.

A bunch of half-carved red colored beetle-shaped figurines.

“Red colored” to “red-colored”
Is “a bunch” the best word choice for this?

A note, written in Old Castilian, that roughly translates to: "If you're not careful and [TEXT DAMAGED] wrong areas, you'll end up in the Endless, where it's nothing but the stench of stagnant water, the madness of [TEXT DAMAGED], and the continuous background noise of burning torches, and [TEXT DAMAGED] of haphazardly segmented empty chambers to be trapped in. God save you if you hear something wandering around nearby, because it sure as hell has heard you…". The rest of the text is illegible.

Remove comma after “nearby”
Now, this is cool. I think I get the concept now. This place is like a medieval version of The Halls. It even mirrors the original text post. At least I think that’s what this is supposed to be!

All methods of communication and navigation do not work properly within the reaches of the Crumbling Dungeons. Returning only static in the case of communication devices and inconsistent signage in the case of gadgets built for the purpose of facilitating navigation through the Backrooms. On top of that, reports of encountering the level have become more and more scarce over the time, which has made studying its properties and characteristics increasingly difficult. Most scholars believe that this is due to The Halls somehow integrating the Crumbling Dungeons into itself, and that in the near future there will no longer be a level to document.

This is great. I could feel myself slowly fading out of existence just as the dungeon.

Although there is no active community on the level at present, the remnants of multiple settlements may be encountered infrequently. These are more akin to barracks than to spaces created with the primary purpose of serving as living spaces. The last attempts to form a bastion against the siege of the unknown. Occasionally, rusted and deteriorated remains left by those who inhabited these barracks can be found. These are the only proofs that these spaces were inhabited. If you are lucky, the objects may be intact enough to be useful. Artifacts that have been recovered from these habitations range from broken spears and rusted swords to pieces of armor and leather clothing. Surprisingly, the remains of those who inhabited the chambers have never been encountered. Their bones lost forever.

“Their bones lost” to “their bones are lost”?
This is pretty cool and introduces some background information that we will not get to understand any better. We are stuck with what is lost and that is it.

Even though most of the entrances to the Crumbling Dungeons are found in The Halls, its erratic nature appears to allow entry from other nearby levels. Entrances to the Crumbling Dungeons take the form of tunnels carved in a reddish stone similar to that constituting the bricks of the level. Occasionally, these tunnels will not be easily noticeable, and it will be necessary to break through walls that are visibly deteriorated in order to access them.

I see nothing wrong here!

As of 2047, it has been 50 years since the last sighting of an entrance to the Crumbling Dungeons, and 53 years since the last reported successful expedition. These last expeditions experienced a level of precariousness that bordered on the safety limits established for an exploration with minimum safety levels. Over the years, the state of chaos in the level only increased and the distances from its entry points and its "limit" decreased. According to estimates based on the statistical models made by the Scholars, the level known as the Crumbling Dungeons relinquished its last portion of terrain about 23 years ago. Accordingly, the Archivists' Council has unanimously decided to declare it defunct.

The sense of timelessness here is amazing. Great way to close as if it were a story!