SCP Foundation

If any entities were to escape from SCP-XXXX they are to be shot on sight.

It's "are to" in this case.

SCP-XXXX is what looks like a wooden shack on the exterior located in ██████, England.

Tone issue. Avoid "looks like/appears to be" unless there is doubt that what the SCP looks like is not, in fact, what it looks like. So, "SCP-XXXX is a wooden shack, located in ██████, England." is better.

SCP-XXXX was found by the foundation after reports surfaced of people entering the building but not coming back out.

Always captialize the word "Foundation" when you refer to the Foundation. Second, something like that is too common for the Foundation to investigate.

It was thought to be a myth until a group of police officers were sent to the site but never came back.

If the police thought it was a myth, why did they bother investigating in the first place? Second, it seems like the foundation woukd only get involved after a poluce investifation.

I'd suggest you replace the above with "SCP-XXXX was discovered by local police after reports involving missing people. The Foundation became aware of SCP-XXXX after the responding officers failed to report their findings."

The interior of SCP-XXXX is where anomalous properties take place.

It should be clear from the description how and where the anomaly occurs, so this can be removed.

The interior of SCP-XXXX, which will now be referred to as SCP-XXXX-1, unless the subject suffers from a phobia relating the size of a room SCP-XXXX-1 will be twice as large as the exterior of SCP-XXXX.

First, the interior doesn't need its own subdesignation. Second, thIs is confusing to read. "Typically, the interior of SCP-XXXX is twice as large as its exterior" is better.

When a subject enters SCP-XXXX, SCP-XXXX-1 will begin to create entities which will be referred to as SCP-XXXX-2.

Ok, the entities do need to be a subdesignation, in this case as SCP-XXXX -1. Just wanted to be clear here.

SCP-XXXX can recognize almost all phobias

What phobias can it not recognize?

If the subject has no phobia related to living organisms, SCP-XXXX will create SCP-XXXX-2 as anything that can kill the subject.

This information probably can be better incorporated into the paragraph. "The interior of SCP-XXXX will alter itself or generate instances of SCP-XXXX-1 to match the phobias of any subjects that enter" would be better.

SCP-XXXX will slowly leach the subject's fear as SCP-XXXX-2 is around. Once SCP-XXXX is satisfied with it's meal, the subject will be eliminated by SCP-XXXX-2. Once the subject has been eliminated, SCP-XXXX-1 will prepare for it's next pray.

First, it's "leech" and "prey". Second, you gave its/it's confusion. "It's" is short for "it is", while "its" indicates possession. Third, how do we know it feeds on fear, or that it becomes satisfied after a certain point? Fourth, how does it prepare itself? And how does it eliminate anyone who goes inside?

Finally, if it flat-out eliminates people that go inside, how do we know anything that happens once someone enters? Unless they have radios, or sonehow escape, they can't tell us what's inside.

Addendum: Tests relating to SCP-XXXX and SCP-XXXX-1 have been made.

This is completely worthless information. What kibd of tests have been done, or tye results of the tests? Just saying that a test has been done isn't all that helpful.

Destruction Test:

SCP stands for "Secure, Contain, Protect" not "Squish, Crush, Pummel". Why would you blow a hole in the side of the SCP? What if that hole didn't close? What if the SCP exploded and levelled the countryside? As far as the Foundation knew, both were equally possible. All things considered, that was an ill-conceived test.