Pirithous had heard rumors about Theseus' courage and strength in battle but he wanted proof. He drove Theseus' herd of cattle from Marathon and Theseus set out to pursue him. Pirithous took up arms and the pair met, then became so impressed by each other they took oaths of friendship. He was a son of Dia or Ixion and Zeus.
They helped hunt the Calydonian Boar. Later, Pirithous was set to marry Hippodamia (offspring: Polypoetes). The centaurs were guests at the party, but they got drunk and tried to abduct the women, including Hippodamia. The Lapiths won the ensuing battle.
Theseus and Pirithous pledged to marry daughters of Zeus. Theseus chose Helen and together they kidnapped her and decided to hold onto her until she was old enough to marry. Pirithous chose Persephone. They left Helen with Theseus' mother, Aethra and travelled to the underworld, domain of Persephone and her husband, Hades. Hades pretended to offer them hospitality and set a feast; as soon as the pair sat down, snakes coiled around their feet and held them there or the stone itself grew and attached itself to their thighs.
Heracles freed Theseus but the earth shook when he attempted to liberate Pirithous. When Theseus returned to Athens, the Dioscuri had taken Helen and Aethra back to Sparta. Pirithous remained in Hades for eternity.
Homer, Odyssey XXI, 295-305, XI, 631; Apollodorus, Bibliotheke; Ovid, Metamorphoses, XII.