Grial (Ciclo Pendragon, 5)

Evil has infiltrated the court in the guise of a beautiful maiden. Unbeknownst to Arthur, his loyal warriors, his beloved Gwenhwyvar, and even the bard Merlin, malevolent, soulless, and cunning powers have seduced the monarch's most faithful champion. And in an inconceivable moment of carelessness, the Grail vanishes, lost in the unknown world of darkness.