Aquila and the Sphinx

From the back cover of the Del Rey first edition: Aquila on the warpath - Equus Insanus, son of Aquila, had been missing for three years on his quest to bring the vile Time Criminal to justice. His Roman half-brother Lucius, nephew to Caesar, feared him dead - until he began receiving strange messages from the lost Lacoti warrior. Then Lucius learned that Equus had been captured by the Time Criminal to be used as the instrument in the utter destruction of their world. Upon discovering that the secret to Equus's salvation lay in the pyramids of Giza and the riddle of the Sphinx, Aquila and Lucius set out for Egypt. They had no idea that the answers they found there would lead them across the entire Roman Empire, through space, and ultimately through time itself - to the final confrontation with the evil Time Criminal and the battle for a man's soul...