“This masterpiece … depicts Titan Prometheus in agony as he is devoured by a vicious eagle. According to the Greek myth, Prometheus stole fire on Mount Olympus from the gods to give to humanity, so Rubens chose to show the moment in which the king of the Olympians, Zeus, ordered he be chained to a rock and have his liver eaten by an eagle every day. Rubens created this work together with the artist Frans Snyders.
“According to the Philadelphia Museum of Art, where this masterpiece is held, the asymmetrical composition was inspired by Titian’s picture of the giant Tityus (1548-49) and the figure of Prometheus was derived from Michelangelo’s prototypes.”
explanation given by artdevivre.com