Anna Skellern is definitely underused in this movie. Just wanted to write that upfront. But it's not a movie about her. It's a movie about an Asylum (as an alternate title does suggest, original title being "The Incident") and the people within. It's also a movie about madness and reality shifting (sort of).
I couldn't really start to explain what the ending is. Apart from the fact it that it would be a spoiler, it's one of those endings that will split the audience. Some will find it amazing, what the director did there and some will think he's full of himself. One thing is for sure: Up until that moment (or rather moments), the movie is pretty straightforward and has an edgy feel to it. And it is pretty good in scaring you (visually with some hard to stomach violent scenes, so not for the faint hearted then), too. If that is something you like, then go ahead and give it a try