Peirithous (Ancient Greek: Πειρίθοος / Πειρίθους; Latin Pirithous) was king of the Lapiths of Larissa in Thessaly, son of Dia by Ixion or Zeus, and the sworn friend of Theseus. His wedding to Hippodamia became the Centauromachy; with Theseus he abducted young Helen and then tried to carry off Persephone from Hades, where he remained bound while Heracles later freed Theseus alone.
Adapted from Wikipedia, "Pirithous" · CC BY-SA 4.0
I.Family and friendship with Theseus
His mother is "heavenly" Dia; his father is either Ixion or Zeus. By Hippodamia (daughter of Atrax or Butes) he fathers Polypoetes, a Greek leader at Troy. Homer's Nestor ranks Peirithous and Theseus among an earlier generation who destroyed a savage mountain tribe — an allusion listeners already knew.
Wanting proof of Theseus's fame, Peirithous rustles cattle from Marathon; when they meet in arms they are so impressed by each other's courage that they swear friendship. Both hunt the Calydonian Boar.
II.Centauromachy
At Peirithous's wedding the invited Centaurs, drunk on wine, try to abduct the Lapith women; Eurytion (or Eurytus) carries off the bride. The Lapiths win the ensuing battle — the Centauromachy, a favorite theme of Greek art from the Parthenon metopes to Roman sarcophagi.
III.Helen, Persephone, and Hades
After Hippodamia's death the friends swear to marry daughters of Zeus. Theseus chooses Helen of Sparta; they abduct her as a child and leave her with Aethra at Aphidnae. Peirithous demands Persephone. Bound by oath, Theseus descends with him to the Underworld; they sit on a rock and cannot rise when the Erinyes appear.
During his twelfth labour Heracles frees Theseus, but the earth shakes when he tries to free Peirithous — the crime of seeking a great goddess as bride is too great. A scholium on Aristophanes records a lost Euripides version in which Hades feeds Peirithous to Cerberus. Attic comedy joked that Theseus left his buttocks stuck to the rock (Hypolispos). Meanwhile the Dioscuri recover Helen and take Aethra and Peirithous's sister Physadeia as handmaidens. Athens later worshipped Peirithous as a hero beside Theseus.