Imagery is good. It would really be supported by a suitably creepy picture. Given that you have a painting in the comments, perhaps use that and label it "a soldier's depiction of the entity"?
The concept is not particularly innovative. "Creature that makes people vanish" and "horseman of the apocalypse" are both common themes. This doesn't necessarily mean the entry is bad in itself, but it does place a higher burden on the execution to carry it.
Focusing on reports of an SCP from a certain period in history is a novel idea. It's good to have a different type of protagonist from the usual researchers, MTF teams and D-class. Showing it from the eyes of someone in the early 1900s helps construct that atmosphere of horror.
However, personally for me, the journals are a start, but they don't do enough. Perhaps because by the time the reader gets to them, we already know how the creature functions due to the extensive description. Perhaps it's also because they only give a glance into the monster and don't have a sting or payoff.
My suggestion would be to reduce the 'tell' of the monster and its abilities in the description and focus on the 'show' in the journals. You could either flesh out the existing plot of the Sommes soldier some more or you could include different accounts from other time periods.
For now, -1.