SCP Foundation

SCP-Z is a collection of objects and machines that were created by a being that calls itself Ankou,

And how would we (the reader and in-universe researcher) know who/what created these items? Are they sentient and claim as such? Is there a label, stating its maker? Or is there a legend describing these items?

store objects created when a death results from using SCP-Z

You may want to explain what are these "objects", as in how do they look like.

-1a immaterially passes through the subject.

According to Dictionary.com, "immaterial" typically means unimportant. Of course, "incorporeal" is also a legit meaning for the word, but not as well-known.

Take it as you will, though I suggest a bit of re-phrasing. Say, "SCP-Z-1a is intangible when cutting a human…"

P.S. I think you should write out all sub-designations in full, as in "SCP-Z-1a" instead of "-1a". It comes as slightly unprofessional, and the 2 terms mean different things. For example, "SCP-Z-1a" specifically refers to a certain item while just "-1a" could refer to "SCP-X-1a" or "SCP-Y-1a".

Study of SCP-Z-1a immediately after usage on a subject has found traces of a substance that resembles the gas that SCP-158 removes from its subjects.

This cross-linking feels like a shout-out to SCP-158 while not adding value to your article. I mean, the reader can already guess it is a soul-stealing item here. Unless you want to imply that SCP-158 and your SCP are grounded in similar concepts… which I felt it won't stand out much…

when the fade and vanish from sight.

Nitpicking: But it is "they", right?

Ankou appears to be a dark being, completely covered in black attire, a robe and a large hat. The face of the being is covered by the hat.

If you're gonna make it a sub-designation of SCP-Z, maintain the proper terminology and call it "SCP-Z-1e".

The manifestation then speaks to the subject…

The ellipsis comes as unprofessionally written for the in-universe scientific document. Instead, you can try things like "Below is a transcription of a manifestation of SCP-Z-1e"

an instance of SCP-Z-1e/2e/Ankou

You can just say "instance of SCP-Z-1e." It has already been defined in the article.

Pros:

• Interesting as to how it modifies its tools over the times and the references to the Isles.

Cons:

• The items are described rather plainly. There is little hook to grab the reader's attention. I would say that it suffers from outright telling the readers what is the thing from the start. You can leave it towards the ending, where you imply or hint at its creator.
• Frankly speaking, Ankou's speeches could work better as interview logs. While it is intriguing for it to be talking about aspects of human history, what does it all add in the greater scheme of your story?