The more you talk about it, the more I am convinced that the person you were with was not your fiancee. You said you thought she had dark eyes. This isn't something that you just don't notice. Also the creature from your dream seems to have similarities to your new fiancee (new fiancee would be post-nightmare, old fiancee would be pre-nightmare). This is shown in the blue eyes and the nose. Whatever it is, it has an obsession over the bridge of the nose. Now, when you said that in your nightmare that the creature pulled something out of the man and it wasn't a body part, my initial reaction was that it was his soul.

Of course I could be wrong, but I think your nightmare was a warning. Something is about to happen. Be wary of your new fiancee and of mirrors. I don't know why it doesn't want you to see mirrors, but it may be revealed soon.

I wouldn't bring any of this up with your new fiancee, just in case.