The Scholar, the Sphinx, and the Fang of Fenrir - A. R. Cook - Bog

David thought his days of fighting monsters were over.
After two years of peace with his wife in Paris, a new danger has emerged. The world-devouring creature Fenrir has broken free of his prison, intent on finding Acacia and taking back what she possesses.
That isn't the only evil afoot...
Rumors abound that an old adversary has returned, and madness itself seeks to claim David. Yet those enemies are nothing compared the new incarnation of Nyx, who will stop at nothing to use Fenrir's abilities to destroy the world.
With time running out, David must band together with old friends and new allies to save both the human world and the world of myth and legend.