Leonardo and Riario have to survive a series of grueling tests before they can enter the Vault of Heaven. Leonardo is bitten by a poisoned snake, and Riario must find the antidote while being hunted by warriors. Ima has sex with the feverish Leonardo, who experiences a series of visions while drugged by the poison. In his dreams he sees his masterpiece, the Mona Lisa, and meets himself as an old man, who reveals that the enemy stalking Leonardo and the Sons of Mithras is known as the Labyrinth. Meanwhile in Naples, Lorenzo is also having to face a life-or-death trial. The deranged King Ferrante places Lorenzo on a gibbet with a hangman's rope round his neck. Lorenzo must kill a galloping horse tied to the other end of the rope with a single bow shot. If he fails, he will be hanged. On the road to Constantinople, Lucrezia encounters al-Rahim, who offers advice in the form of the ghost of Lucrezia's long-dead sister, Amelia.
