Firstly, I want to address what I thought had been a contradiction in the previous book. Although Nadia cannot be heard, seen or smelled when cloaked, this book reveals that she will leave a scent trail as she moves about. So that explains that.
Nadia's time in an Eternity Crucible has given her enough magical power that Lord Morviland deems her adequate to fulfill his bargain with the Forerunner. I believe the Forerunner is who decided the timing as he is manipulating the Rebels and their plan is coming to fruition. Nadia must obtain three items designated by Nicholas Connor, her former lover and head of the Rebels, who incidentally would like nothing better than to see her dead. Nadia's reciprocates the feeling.
She has a longer discourse with Lord Morviland than we've ever seen, even flat out disagreeing with and insulting him. Apparently the severe psychological damage and numerous deaths have inured her fear of him somewhat. But, as we all know by this time, she is helpless to refuse him if she wants to keep her brother alive.
Nadia works with Nicholas and his crew to obtain a briefcase that had been interred in the tomb of a former Rebel leader. This involves entering the ruined city of Chicago filled with undead and guarded by a myothar.
She manages to overcome all obstacles, including a not totally unexpected betrayal by Nicholas, to live to fight another day.