Something that really elevates the Not! Them’s creepiness level for me is what Gertrude says in “The Kind Mother.”
“I have destroyed the tape Ms. Cooper provided us as a precaution. I have no interest in attracting this thing’s attention.”
Gertrude Robinson is old women in a career where most people die young. She’s stopped multiple mass ritual’s and has survived a ritual herself when she was almost burned alive accidentally connecting herself to Agnes. She’s easily the toughest person in the series.
Yet, she has so little interest in attracting the Not! Them’s attention that she destroys knowledge just to stay under it’s radar. That’s not a small deal for someone attached to The Eye, or for someone who’s usually up for taking down a fear.
I think her reluctance speaks volumes for how dangerous The Stranger’s powers of replacement are. If it replaced someone in The Institute even Gertrude would have no way of knowing. And if it replaced her, then it would have free rein of The Archives.
Neither of those scenarios would be good and Gertrude understands that enough to turn a blind eye to the knowledge she’s given in the gift of a tape. That makes the Not! Them creature so much scarier for me, and it’s already a pretty horrifying concept.