In the final installment of the Enigma series, Surina and Dominic descend into the tunnels of Wupatki to rescue Hannah Hale. Their plans are tripped up by an unexpected tagalong, and Surina is uneasy about Hannah's sympathy for Tagua, a demon sorcerer of the underworld. Hannah assures them that they can return home if they set things right in the zodiac and learn the secret of Ophiuchus-but is she willing to return alone, or will she unwittingly release Tagua into their world?