The Prometheus Unbound, the loss of which we lament more almost than that of any other tragedy, although many considerable fragments of it remain, began at a totally different period of the world [than the Prometheus Bound]. Prometheus, however, still remains bound to the rock in Scythia, and, as Hermes had prophetically threatened, he is daily torn by the eagle of Zeus. The chorus, instead of the Oceanides, consists of Titans escaped from the durance of Tartarus. Aeschylus, therefore, like Pindar, adopts the idea, originating with the Orphic poets, that Zeus, after he had firmly fixed the government of the world, proclaimed a general amnesty, and restored peace among the vanquished powers of heaven. Meanwhile mankind had arrived at a much higher degree of dignity than even Prometheus had designed for them, by means of the hero-race, and man became, as it were, ennobled through heroes sprung from the Olympic gods. Hercules, the son of Zeus by a distant descendant of Io, was the greatest benefactor and friend of man among heroes, as Prometheus was among Titans. He now appears, and, after hearing from Prometheus the benefits he has conferred upon man, and receiving a proof of his good will in the way of prediction and advice with regard to his own future adventures, releases the sufferer from the torments of the eagle, and from his chains. He does this of his own free will, but manifestly by the permission of Zeus. Zeus has already fixed upon the immortal who is ready to resign his immortality. Cheiron is, without Hercules' intending it, wounded by one of the poisoned arrows of the hero, and, in order to escape endless torments, is willing to descend into the lower world. We must suppose that, at the end of the piece, the power and majesty of Zeus and the profound wisdom of his decrees are so gloriously manifested, that the pride of Prometheus is entirely broken. Prometheus now brings a wreath of Agnus Castus, and probably a ring also, made from the iron of his fetters, mysterious symbols of the dependence and subjection of the human race; and he now willingly proclaims his mother's ancient prophecy, that a son more powerful than the father who begot him should be born of the sea-goddess Thetis; whereupon Zeus resolves to marry the goddess to the mortal Peleus.
It is scarcely possible to conceive a more perfect katharsis of a tragedy, according to the requisitions of Aristotle.
The passions of fear, pity, hatred, love, anger, and admiration, as excited and stirred up by the actions and destiny of the individual characters in this middle piece [of the trilogy], produce rather a distressing than a pleasing effect; but under the guidance of sublime and significant images they take such a course of development, that an elevated yet softened tone is shed over them, and all is resolved into a feeling of awe and devotion for the decrees of a higher power.
[In 1820, Percy Bysshe Shelley published a four-act play re-imagining the lost Prometheus Unbound of Aeschylus. However, unlike Aeschylus' version, there is no reconciliation between Prometheus and Zeus in Shelley's drama. Instead, Jupiter/Zeus is overthrown.]