Wrath of the Storm (Mark of the Thief #3)

Nicolas Calva has Caesar's magic bulla, the Malice of Mars, and he has hidden the dangerous Jupiter Stone, but he still has to find a way to defeat The Mistress, a dragon who contains the soul of the vestalis Atroxia, and who hates Rome, and save his mother and friends, and maybe even the Empire itself--and he is troubled by his sneaking sympathy for Atroxia, who was unjustly condemned.