Acheron in Greek and Roman Biography and Mythology
（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. - A dictionary of Greek and Roman biography and mythology William Smith, Ed.
Acheron in Wikipedia
In ancient Greek mythology, Acheron was known as the river of pain, and was one of the five rivers of the Greek underworld. In the Homeric poems the Acheron was described as a river of Hades, into which Cocytus and Phlegethon both flowed. The Roman poet Virgil called it the principal river of Tartarus, from which the Styx and Cocytus both sprang. The newly-dead would be ferried across the Acheron by Charon in order to enter the Underworld. The Suda describes the river as "a place of healing, not a place of punishment, cleansing and purging the sins of humans." According to later traditions, Acheron had been a son of Helios and either Gaia or Demeter, who had been turned into the Underworld river bearing his name after he refreshed the Titans with drink during their contest with Zeus. By this myth, Acheron is also the father of Ascalaphus by either Orphne or Gorgyra.The river called Acheron with the nearby ruins of the Necromanteion is found near Parga on the mainland opposite Corfu. Another branch of Acheron was believed to surface at the Acherusian cape (now Eregli in Turkey) and was seen by the Argonauts according to Apollonius of Rhodes. Greeks who settled in Italy identified the Acherusian lake into which Acheron flowed with Lake Avernus. Plato in his Phaedo identified Acheron as the second greatest river in the world, excelled only by Oceanus. He claimed that Acheron flowed in the opposite direction from Oceanus beneath the earth under desert places. The word is also occasionally used as a synecdoche for Hades itself. Virgil mentions Acheron with the other infernal rivers in his description of the underworld in Book VI of the Aeneid. In VII, line 390 he gives to Juno the famous saying, flectere si nequeo superos, Acheronta movebo: 'If I cannot deflect the will of Heaven, I shall move Hell.' The Acheron was sometimes referred to as a lake or swamp in Greek literature, as in Aristophanes' The Frogs and Euripides' Alcestis. In Dante's Inferno, the Acheron river forms the border of Hell. Following Greek mythology, Charon ferries souls across this river to Hell. - Wikipedia