The next adventure in which Theseus engaged was to lead the Athenian army against Pirithous, king of the Lapithæ, who had been secretly carrying off some of his herds. Full of anger at these continued plunderings, Theseus came suddenly upon the marauders as they were boldly making off with their booty; but as soon as he and Pirithous were brought face to face, they were seized with such an admiration for each other that they had no longer any feelings of enmity. They threw down their weapons, clasped each other's hands, and swore eternal friendship. To prove his devotion to his new friend, Theseus agreed to accompany him to the court of Adrastus, king of Argos, and to be present at the marriage of Pirithous to Hippodamia, the daughter of the king. Many guests came to the wedding, and among them were Hercules and a number of the centaurs. The beauty of the bride so won the admiration of the centaurs that they determined to kidnap her, and a terrible battle ensued between them and the Lapithæ, who were aided by Theseus and Hercules. Finally the centaurs were driven away, but not before many of them had been killed or wounded. The bride who had been the cause of this strenuous fighting did not live long, however, and Pirithous soon found himself like Theseus, a disconsolate widower.
Having been so unfortunate in their wives, the two heroes determined not to seek again any king's daughters in marriage, but to choose some one of divine parentage. So Theseus decided upon Helen, the child of Jupiter and Leda[105] whom the ruler of the gods wooed in the form of a snow-white swan. Pirithous's choice fell upon Proserpina, the wife of Pluto. Theseus succeeded in carrying off Helen; but as she was at that time a mere child, he left her in the care of his mother Æthra until she should be old enough for marriage. Meanwhile Helen's twin brothers, Castor and Pollux, having discovered who her abductor was, waited until he was absent on the venture with his friend Pirithous, and then went to the rescue of their sister, whom they took forcibly from Æthra, and carried triumphantly home to Sparta.
Of these twin brothers, Castor was mortal, and some time later was slain in battle. Pollux, who was immortal, then begged Jupiter to let him die also, that he might not be separated from his brother. The ruler of Olympus, touched by this evidence of devotion, allowed Castor to return to life on condition that Pollux would spend half of his time in Hades. Later on both brothers were translated to the heavens, where they form a bright constellation, one of the signs of the Zodiac.[106]