Mark of the thief trilogy

1.

When Nic, a slave in the mines outside of Rome, is forced to enter a sealed cavern containing lost treasures of Julius Caesar, he finds himself in possession of an ancient amulet filled with magic once reserved for the Gods, and becomes the center of a conspiracy to overthrow the emperor and destroy Rome.

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.