This article's factual accuracy is disputed. (May 2011)
As Zeus Meilichius or Meilichios, the Olympian of Greek mythology subsumed as an attributive epithet an earlier chthonic daimon, Meilichios, who was propitiated in Athens by archaic rituals, as Jane Ellen Harrison demonstrated in detail in Prolegomena to the Study of Greek Religion (1903). In the course of examining the archaic aspects of the Diasia festival, the greatest Athenian festival accorded Zeus, she showed that it had been superimposed upon an earlier propitiatory ceremony.
Explicitly inscribed votive reliefs show that Meilichios was figured in the form of a serpent, who might be invoked as a kind of Ploutos, bringer of wealth. He had some of the avenging and fearful character of an Erynis, for Pausanias saw near the River Cephissus "an ancient altar of Zeus Meilichios; on it Theseus received purification from the descendants of Phytalos after he had slain among other robbers Sinis, who was related to himself". Meilichios' sacrifice was a holocaust, which was wholly consumed in fire and not shared by the votaries, "a dread renunciation to a dreadful power" (Harrison, p. 16), in nocturnal rites performed in an atmosphere of "chilly gloom" (Harrison), that was rendered in Greek as stygiotes.
Zeus tended to obscure the earlier figure he had supplanted. An Athenian of the fifth century would likely have conceived Zeus Meilichios as Zeus-Hades, Zeus "in his chthonic aspect".