Pirithous was born in Thessaly, the wild northern region of Greece famous for its horses, its mountains, and the strange creatures that roamed its slopes. His father was Zeus, who had come to his mother Dia, the wife of the mortal king Ixion, in the form of a stallion, a union that produced a child of extraordinary strength and recklessness. Some said that Ixion, discovering the deception, had tried to kill the infant Pirithous by throwing him to the wolves, but the child was rescued by shepherds and raised among the Lapiths, the tribe of horsemen who dwelt in the mountains of Thessaly. Whether this story is true or merely a later invention to explain his wild nature, Pirithous grew to be a king of great courage and even greater impetuosity, a man who never saw an adventure without wanting to join it.

His most famous friendship began with Theseus, the king of Athens who had slain the Minotaur, the half-man half-bull creature that lived in the labyrinth built by Daedalus on the island of Crete. The two met as enemies, or so one story tells it. Pirithous, hearing of Theseus's fame, decided to test him by driving off his cattle from Marathon, the plain near Athens. Theseus pursued him, armed and furious, but when the two faced each other they were so struck by each other's courage that they dropped their weapons and swore an oath of friendship instead. From that day forward they were inseparable, bound by a bond stronger than blood, and every adventure that one undertook, the other joined. They hunted the Calydonian Boar together, the monstrous beast sent by Artemis to ravage the land of Calydon. They fought side by side in a dozen minor wars. And when Pirithous decided to marry, Theseus was there to stand beside him.

The bride was Hippodamia, a woman of such beauty that her name became legendary. Pirithous invited the centaurs to his wedding, a gesture of kinship and reconciliation, for the centaurs were also children of Ixion, born from the union of Ixion and the cloud-phantom Nephele, and thus related to the Lapiths by blood. The wedding was held in a great hall on Mount Pelion, and the centaurs arrived as honoured guests, bringing gifts and good wishes. But the wine flowed too freely, and the centaurs, who had never learned to control their appetites, became drunk. One of them, Eurytion or Eurytus, seized Hippodamia by the hair and tried to drag her away. The other centaurs followed his example, grabbing the women of the wedding party and attempting to carry them off into the mountains. Pirithous and Theseus sprang to their feet, drew their swords, and the battle that followed was called the Centauromachy, the war of the centaurs and the Lapiths. The fighting was savage, and the centaurs, though strong, were no match for the organised fury of the Lapiths and their Athenian allies. Many centaurs were killed, including Eurytion, and the survivors were driven from the hall. The wedding continued, but the memory of the violence never faded. In Greek art, the Centauromachy became one of the most popular subjects, carved into the metopes of the Parthenon in Athens and painted on hundreds of vases, a permanent reminder that the boundary between man and beast was thin, and that wine could turn kin into enemies in a single cup.

But Pirithous was not satisfied with being a king and a husband. His divine blood and his friendship with the greatest hero in Greece had made him ambitious, and ambition, in Greek myth, was the quality that most often led to ruin. He decided that he wanted a wife worthy of his stature, and no mortal woman would do. He looked to the underworld, to Persephone, the daughter of Demeter, the goddess of the harvest, who had been abducted by Hades and made queen of the dead. Persephone was not merely beautiful; she was divine, and Pirithous wanted her for his own. He went to Theseus and reminded him of their oath: they had sworn to help each other in any adventure, no matter how dangerous. Theseus, bound by his word and perhaps flattered by the scale of the plan, agreed. Together they descended into the underworld, the realm of Hades, the god who ruled over the dead, a place from which no living man had ever returned.

The underworld was not the hell of later Christian imagination but a dark and shadowy place, divided into regions for the virtuous dead, the wicked dead, and the ordinary dead who had done neither great good nor great evil. Hades ruled it with his wife Persephone, and the three-headed dog Cerberus guarded the entrance. Pirithous and Theseus made their way past Cerberus, either by stealth or by strength, and approached the throne of Hades. They announced their intention to take Persephone back to the world of the living. Hades, the god of the dead, did not strike them down with thunderbolts or summon armies of ghosts. Instead, he smiled, and offered them seats. The seats were magical, chairs of forgetfulness that bound whoever sat in them to the spot, paralysing their bodies and clouding their minds so that they could neither rise nor remember why they had come. Pirithous and Theseus sat, and the moment they did, they were trapped. They sat in the house of Hades for months or years, unable to move, unable to speak, forgotten by the world above and ignored by the dead around them.

Their rescue, if it happened at all, came from Heracles, the hero forced to complete twelve impossible labours as penance for killing his wife and children in a fit of madness. His twelfth labour was to fetch Cerberus from the underworld, and when he descended into Hades he found Theseus and Pirithous still bound to their chairs. Heracles, with his enormous strength, seized Theseus and tore him free, though some said that a piece of his buttocks remained stuck to the chair, which is why Athenian men were said to have small buttocks. But when Heracles tried to free Pirithous, the earth shook, and a voice warned him that Pirithous's crime was too great to be pardoned. Heracles, who was brave but not foolish, left Pirithous where he sat and departed with Cerberus and Theseus. In other versions of the story, Pirithous remained in the underworld forever, a prisoner of his own ambition, while Theseus returned to Athens and resumed his kingship. In still other versions, both heroes were freed, but these are less common and less believed.