Halo 3, what does cortana talk about?

[Lyssa!]

I have defied gods and demons. I am your shield; I am your sword. I know you; your past, your future. This is the way the world ends

what does that quote mean

 

[Jacob W]

She has not done what she was asked from what she calls gods, she protects him, she knows everything about him, his past and future. She is going to be a part of the worlds destruction.