Situated close to ancient Ephyra, on the top of a small hill, on the northwestern side of which lies the present day village of Mesopotamos, is the most important and most ancient Oracle of the Dead of Antiquity, the Nekyomanteion (nekys, dead) of Acheron.


It was here that the ancients located the Gates of the Underworld which led to the kingdom of Hades (Pluto), who ascended to the Upper World only to abduct the beautiful Persephone. Charon, the ferryman of Hades carried dead souls across the Acheron river to the entrance of Hades, after having received a coin (the obol) which was placed in or on the mouth of the dead by their relatives to pay for passage. Lucian, a satirist author of the second century AD gave a depictive description of the dead entering Hades in his work 'Dialogues of the Dead': ('Charon: Abominable fellow, pay up the fare. Dead: You can't get it from one who doesn't have it...'). Upon the bank of the river Styx stood Cerberus, the guardian three-headed hound with a lion tail that ended in a serpent (its figure has been depicted on the coins of the Thesprotian city, Elea). Upon entering the Underworld, the souls presented themselves to the four judges, Pluto, Minos, Rhadamanthys and Aeacus who decided over their deeds during their life on earth. Only few mortals managed to descend to Hades alive and come back to life again. The cunning Odysseus, as related by Homer in the Odyssey, was the first one who, following the advice of the sorceress Circe, succeed in entering to collect the relevant information that would help him to return to Ithaca and returned alive. It was also while descending that he heard about the loss of his mother, Anticleia, who had died in the meantime. This difficult endeavour, according to mythology, was also attempted by Hercules, Theseus and Orpheus, the king of Thrace, who tried to bring his wife Eurydice back.

The alluvial deposits of the rivers Acheron, Cocytus (Mavros) and Pyriphlegethon (Vovos), contributed to the formation of a marsh and later a lake, the Acherusia, which today has dried up. There is a rock with a cave on the northwest side of the lake at the meeting point of the three rivers which was an ideal location to erect the Oracle. A renowned place, also known through Homer's eloquent descriptions (eighth century BC), for centuries a popular destination where mortals turned in order to meet the dead, bring proper offerings, and predict things to come.

The physical and spiritual tests experienced during their stay for days on end in the dark rooms of the Necromanteion: the isolation, the magical rituals, the prayers, the summonings, the wandering through dark corridors and the common belief in apparitions of the dead created the necessary psychic predisposition in the pilgrim. The special diet which the pilgrims were required to have contributed to this considerably. The main sanctuary is rectangular with 22 m long sides. It comprises the main hall, corridors and reception rooms, and for the staff, preparation rooms, storing rooms where clay jars with visitors' offerings have been preserved, the labyrinth and the actual sanctuary where the oracles were given.

The oracle, known to everyone of Hellenic origin since Homer's epoch, was burnt down and destroyed by the Romans in 167 BC. After its destruction, the courtyard was inhabited again in the first century BC. In the eighteenth century the Monastery of Agios Ioannis Prodromos (Saint John the Baptist) was built upon the ruins of the Oracle, the primary temple (Katholikon) which still stands upon the ruins.