So Listen warns Harry the Genoshwa (spelling may be wrong) is still around, despite Harry being sure he smashed him and trapped him in Hades. As far as we know there is no way he could have gotten out of there. He didn't escape with Nic.
So.... What if Nic is somehow hiding as the Genoshwa?
Oh, and while on the topic, the Genoshwa is essentially Grendel. Interesting then that the Grendelkin that Harry and Gard fought seemed so different. I mean he needed mead and a virgin to procreate. Somehow I doubt Listen has that problem...