They ally with Bran, a Welsh descendant of King Arthur, and the three Drew children, to form the Six who are prophesied to triumph over the powers of the Dark.

Significant mythical elements in the book include the bard Taliesin (under his alternate name Gwion), King Gwyddno Garanhir and the Drowned Hundred, the Welsh tradition of the Mari Llwyd, Arthur's ship Prydwen, and the Ritual of oak and mistletoe.

[1] The Children's Literature Review criticized elements of the book's plot, such as the use of time travel, but praised its setting and descriptive writing.

[2] Kirkus Reviews was critical of the book's climax, saying "to the end the discrepancy between her grand scheme and the particulars of the story is unbridged, giving a morally and intellectually hollow ring to the whole.

"[3] Rebecca Fisher of fantasyliterature.com praised Cooper's subtle writing, but criticized the book's pacing choices and lack of suspense.