In myth, he was King of the Lapiths (a tribe living in Thessaly, in northern Greece), and companion of Theseus in many adventures: seizing Helen, Antiope, an attempt on Persephone. His marriage was the occasion of a battle between the Lapiths and Centaurs (Centauromachy), where Centaurs carried off Lapith women and were fought by the groom, Theseus and the Lapiths, including Kaineus. For their attempt on Persephone he and Theseus were imprisoned in Hades (stuck to rocks) whence only Theseus was rescued by Herakles.
Above: Peirithoos and Theseus at the feast. Detail from South Italian red-figure clay vase, c.4th century BC. London. British Museum F272. Photo. Mus. C2644 © British Museum. Licence Plate 11 UK 1007 181