Prometheus tells Io she will wonder across dangerous lands, including those of the Gorgons. She will eventually cross into Ethiopia, and from there follow the Nile from its source into the land of Egypt. There she will find a place to make a colony for her and her descendents.
Prometheus also speaks about her past wanderings, showing that his prophetic powers include an ability to see the past. He foretells more of Io's future. In Egypt, she will be healed by the hand of Zeus, and bear a son. Fifty women, descended from that son, will be brought to Argos against their will and forced to take Argive husbands. Forty-nine of the sisters will murder their husbands, but the fiftieth will not. From her will come a line of Argive kings. From her line will also come a great archer. Prometheus does not give his name, but the audience would have recognized that he was referring to Hercules.
Io feels the madness coming upon her again, and she leaves, pursued by the sting of the gadfly.
The Chorus delivers an ode and the dangers of unequal marriage. The mortal maidens chosen by Zeus suffer terribly because of Hera's jealousy.
Prometheus shouts out with a kind of maddened glee that Zeus will not be so mighty for long. A new power will come, and Zeus will suffer worse pains than those Prometheus is suffering now. His only hope of avoiding this fate is with Prometheus' aid.
Hermes, messenger of the gods and Zeus's son, arrives as Zeus's spokesman. Though he threatens Prometheus with further tortures, the titan will not say which marriage will bring doom on Zeus. Prometheus is becoming more and more frenzied, determined to face the tortures: he will burned by lightning and fire, locked beneath ground and buried alive. At other times, beasts will tear out his innards. But he will not die. He refuses to give in, and Hermes accuses him of insanity. The Chorus refuses to abandon Prometheus; they will suffer along with him. Prometheus cries, telling us that the tortures are taking place. He cries out to his mother, the Earth goddess Themis, source of all his prophecies: "O Holy mother mine, / O Sky that circling brings the light to all, / you see me, how I suffer, how unjustly" (ll. 1090-2).
Prometheus finishes his description of fate's grand design for Io and her descendents. Fate and the gods working in grand circles is part of Aeschylus' vision. The Oresteia, as the only full trilogy we have, most aptly demonstrates the scope and beauty of this vision. Although fate working towards an end is a key part of The Oresteia and Prometheus Bound, there are differences between how Aeschylus treats fate and design in these two different cycles. Greek tragedians were not constrained to a meticulous or consistent theology/ideology throughout their careers; they were artists, not Catholic scholastics. What's clear is that Aeschylus was fascinated by questions of design, from both a philosophical and religious point of view. Comparison of The Oresteia and Prometheus Bound throws his treatment of fate in the Prometheus story into sharp relief.
CHORUS: And is Zeus, then, weaker than these? PROMETHEUS: Yes, For he cannot escape what is fated. (ll. 517-8).
The locus of fate remains consistent throughout the play. The grand design here is a power above the gods. In The Oresteia, the gods interceded to turn the trials of the House of Atreus into a step forward in the civilizing of gods and men. Perhaps because the central characters of the Prometheus myth are themselves gods, Aeschylus felt the need to remove fate's locus one place higher. Prometheus insists that all prophecies come from his mother, Themis, the earth: she "tells" him what will pass. But because of the specificity of the prophecies, and his apparent ability to pull out appropriate predictions for whomever happens to pass by, Themis seems to be a more abstract than literal source. Themis becomes life-force, earth mother, in an abstract way. She is not as anthropomorphized as the Olympians.
Not even Zeus can thwart fate, unless Prometheus helps him. Prometheus is unique because at critical points, he is allowed the freedom to make choices. Apparently, because of his wisdom, he will be the one who chooses the new ruler of the cosmos. Not powerful enough to rule himself, he is nonetheless the possessor of a bit of strategic knowledge that could save or destroy Zeus. Prometheus is more than a lesser god, secondary to the Olympians. This titan is the intermediary between the rest of creation and the most primal forces of the universe. His genius makes him the emissary between Truth and the rest of us; his role is unique, even among the gods. Apollo may be the prophet of the Olympians, Zeus's mouthpiece, but Prometheus' truth comes from an even higher, more primal power. Prometheus gifts of prophetic insight, craftsmanship, storytelling, and creative power come together to make a powerful metaphor for genius.
In this last section of the play, we see Prometheus in his many roles. He is the great visionary, the Sufferer, the persistent champion of humankind. Aeschylus manages to hold all of the levels of meaning intact. As he scolds Hermes, he brings back the political symbolism: "You are young / and young your rule and you think that the tower / in which you live is free from sorrow: from it / have I not seen two tyrants thrown?" (ll. 950-7). He is both rebel and visionary; thus he can take comfort in the future he knows is inevitable, when his enemies are laid low: "Worship him, pray; flatter whatever king / is king today; but I care less than nothing / for Zeus" (ll. 937-9). Either Zeus, by necessity, will free him, or Zeus's successor will.
But we also see him in a near-maddened state, shrieking out in a way that seems to invite torture. He speaks to Hermes fearlessly: "Be sure of this: when I set my misfortune / against your slavery, I would not change" (ll. 966-7). Later, he shouts out defiantly that "there is no disgrace in suffering / at an enemy's hand, when you hate mutually" (ll. 1040-1). He describes the tortures waiting for him, seemingly proud of what he will suffer. Hermes is appalled: "These are a madman's words, a madman's plan" (l. 1053). Prometheus has become somewhat unhinged. It is the madness of a righteous man facing inevitable injustice; it also comes from Aeschylus' sense of drama, as it brings a kind of climax to this play. But Aeschylus is also showing that the path to madness is not hard to imagine for a misunderstood visionary or genius.
The true climax is Prometheus' refusal, in face of torture, to give in to his punisher. Aeschylus further establishes Prometheus as charismatic visionary, revolutionary even, by having the Chorus stay with him. They will be his friends in suffering, loyal to the titan despite the wrath of Zeus. In doing so, they have allied themselves to the rebel against the king of the gods. Aeschylus is emphasizing Prometheus' power as a visionary and leader.
In the last lines of the play, Prometheus is calling on powers beyond the anthropomorphized Olympian gods. His cries are the Themis, source of all life, and sky, which sheds its light evenly on all. Such forces seem beyond human need or motivations. And those who know Greek myth might recall that at the beginning of time, all beings came from the union between Earth and Sky. These are ancient, primal forces, as abstract and mysterious as fate. For Prometheus, the titan overpowered by the supreme anthropomorphized deity, invocation of still higher, more abstracted powers seems appropriate. He may be subject to the whims of Zeus, but he is still the being with access to forms of absolute truth, absolute right. Vulnerable as he is, and subjected as he is to the indignities of torture and ridicule by beings less wise and beautiful than he, there remains a part of Prometheus that the other gods cannot touch. In man, this characteristic can come in different forms: genius, vision, moral strength. For Prometheus, this power will sustain him and ultimately win him liberation from his chains. The titan will not compromise, and he is speaking the truth when he tells Hermes that he would not trade his slavery for Hermes' status as Zeus's lackey. And by enduring his tortures while withholding the knowledge crucial to Zeus, the god shows that he is still in control of his own destiny. Even as he is chained and tortured, some part of Prometheus remains free.