SCP-XXXX is a cave in the Attica region of Greece.
I think you should describe the cave first, since it has the main designation, not the shadows.
SCP-XXXX-1A, and 1B have never been observed by the Foundation but historical records and the extra chairs suggest they exist.
From Glaucon's letter to Plato, I assume that one of the chair was Stelio's, right? Also the comma behind SCP-XXXX-1A is misplaced.
The sides of the seats make SCP-XXXX-1 instances unable to see anything other than directly in front of it.
I think "placing" is a more suitable word then "side". And with this, you mean XXXX-1 instances remain seated, right? If so, I'd love to see it in the description.
The first two seats are uninhabited
Is there supposed to be a missing full-stop here, or is the sentence cut off? And what about the third seat?
SCP-XXXX displays anomalous properties wherein objects that cannot be physically observed cast a shadow on the wall of the cave.
What do you mean with "cannot be physically observed"? The objects in the testing log can be all seen.
SCP-XXXX-1's shadows are capable of interacting with the anomalous shadows.
What are "the anomalous shadows" here? Aren't XXXX-1 the only thing anomalous here?
SCP-XXXX-1's shadow picks up the ball but it appears to be significantly heavier than the original volleyball.
Phrasing. "SCP-XXXX-1's shadow picks up the ball, which appears to be significantly heavier than the original volleyball."
After an hour, SCP-XXXX-1 begins interacting with a random shadow originating from an unknown source, in which it accidentally steps on the horse shadow.
What would this unknown shadow be?
SCP-XXXX-1 is distressed and suffers cardiac arrest
If the Foundation couldn't see the XXXX-1 instances, how could they tell they were distressed?
Both her she and the intruder passed within the line of sight of SCP-XXXX-1, and disappeared.