Why was she blind at the end?
The previous sentinel was blind. Maybe all the others were, too, but did I miss the part where it's elaborated upon why blindness is a thing and how it comes about?
I could speculate, in the universe of the film, that blindness gives way to a second sight needed to be a sentinel, but in more practical real-world terms, I think that the whole thing was just a visual that the film's producers agreed was creepy.