(Ἀχέρων). In ancient geography there occur several rivers of this name, all of which were, at least at one time, believed to be connected with the lower world. The river first looked upon in this light was the Acheron in Thesprotia, in Epirus, a country which appeared to the earliest Greeks as the end of the world in the west, and the locality of the river led them to the belief that it was the entrance into the lower world. When subsequently Epirus and the countries beyond the sea became better known, the Acheron or the entrance to the lower world was transferred to other more distant parts, and at last the Acheron was placed in the lower world itself. Thus we find in the Homeric poems (Od. 10.513; comp. Paus. 1.17.5) the Acheron described as a river of Hades, into which the Pyriphlegeton and Cocytus are said to flow. Virgil (Aen. 6.297, with the note of Servius) describes it as the principal river of Tartarus, from which the Styx and Cocytus sprang. According to later traditions, Acheron had been a son of Helios and Gaea or Demeter, and was changed into the river bearing his name in the lower world, because he had refreshed the Titans with drink during their contest with Zeus. They further state that Ascalaphus was a son of Acheron and Orphne or Gorgyra. (Natal. Com. 3.1.) In late writers the name Acheron is used in a general sense to designate the whole of the lower world. (Verg. A. 7.312; Cic. post redit. in Senat. 10; C. Nepos, Dion, 10.) The Etruscans too were acquainted with the worship of Acheron (Acheruns) from very early times, as we must infer from their Acheruntici libri, which among various other things treated on the deification of the souls, and on the sacrifices (Acheruntia sacra) by which this was to be effected. (Müller, Etrusker, 2.27, &c.) The description of the Acheron and the lower world in general in Plato's Phaedo (p. 112) is very peculiar, and not very easy to understand.
[L.S]A Dictionary of Greek and Roman biography and mythology
Smith, William
A Dictionary of Greek and Roman biography and mythology. William Smith, LLD, ed. 1890