Since ancient times their name has been spoken only in hushed tones. Cloaked in anonymity, they guard history's greatest and most devastating secret.

In the early twentieth century, when Barcelona was celebrated as the center of modernist art and design, the grand master of an ancient religious brotherhood prepares to die--passing the care of a sacred relic to a prominent member of his order, the revered artist and architect Antonio Gaudi. The relic, an artifact dating back to the early Christian era, could prove disastrous if it were to fall into inappropriate hands--and many...