In the mysterious city of Queen Mab, the forces of light and darkness are locked in mortal combat for the truth. Dedicated agents of the Holy Inquisition battle their hidden enemies, members of the notorious Cognite, to unmask the identity behind the mysterious all-powerful King in Yellow. At the epicenter of the confrontation is the pariah Alizebeth Biquin. Will she side with the Inquisition or will she prefer Cognite, who raised her? If she chooses the Inquisition, will it be the wise but ruthless Ravenor or his rival, the convicted heretic Eisenhorn? Bequin must face an onslaught of angels, demons, and even the monstrous warriors of the Traitor Legions to unravel the greatest mystery of his life. The beloved characters of Eisenhorn and Ravenor return as bitter adversaries in a novel of esoteric mystery, eerie intrigue, and intense action, where the revelation of one's true identity brings with it death... or it could shake the Imperium to its very foundations.