[citation needed] At the time of the book's publication, Kirkus Reviews said, "Cooper is clearly building towards a thumping conclusion in the fifth and next volume and even those of us who have doubts about the significance of all this thunderous moral absolutism will want to get in on the action.

"[9] Karen Patricia Smith has written, "Will is assisted in his quest for a golden harp by several people, including Bran, son of King Arthur, brought forward in time.

Cooper continues her exploration of the many guises of evil and reiterates the theme that the Dark is a wily foe, capable of taking many forms.

"[10] Mary Corran said that "in The Grey King, it is human emotions which are the danger—jealousy, anger and hate, which open mortal minds to the invasion of the Dark.

The fourth book is strongly Arthurian in content ... Cooper deals with the innate antagonism between mortals and immortals impressively, and in The Grey King reaches great heights as she mingles the worlds and peoples of legend and the present day.