What the heck happened to Ciri in S2?

I have not read the books, but isnt Ciri insanely powerful and >!having Elder Blood!< should make her immune to >!possession!< right? Because if that can happen to her that completely messes up her abilities in my head.

It seems like a massive mess-up to allow her to get >!possessed by some random spirit.!<

Was this actually in the books? If so how do the books explain this, because I surely don't get it.