The shrine of Dodona in Epirus

The shrine of Dodona was regarded as the oldest Hellenic oracle and is located in Epirus, Greece
Worth mentioning, even though not within the region but close to the two National Parks of Vikos-Aoos and Pindus, in Epirus, Northwestern Greece, lays Dodona (Doric Greek: Δωδώνᾱ, Dōdṓnā, Ionic and Attic Greek: Δωδώνη), which was an oracle devoted to a Mother Goddess identified at other sites with Rhea or Gaia, but here called Dione, who was joined and partly supplanted in historical times by the Greek god Zeus.

The shrine is only an hour or so away from the Ecoregion of Pindus and was regarded as the oldest Hellenic oracle, possibly dating to the second millennium BCE according to Herodotus. Situated in a remote region away from the main Greek poleis, it was considered second only to the oracle of Delphi in prestige. Priestesses and priests in the sacred grove interpreted the rustling of the oak (or beech) leaves to determine the correct actions to be taken. Aristotle considered the region around Dodona to have been part of Hellas and the region where the Hellenes originated. The oracle was first under the control of the Thesprotians before it passed into the hands of the Molossians. It remained an important religious sanctuary until the rise of Christianity during the Late Roman era.

Though the earliest inscriptions at the site date to ca. 550–500 BCE, archaeological excavations over more than a century have recovered artifacts as early as the Mycenaean era, many now at the National Archaeological Museum of Athens, and some in the archaeological museum at nearby Ioannina. Archaeologists have also found Illyrian dedications and objects that were received by the oracle during the 7th century BCE. Until 650 BCE, Dodona was a religious and oracular centre mainly for northern tribes: only after 650 BCE did it become important for the southern tribes.
At Dodona, Zeus was worshipped as “Zeus Naios” or “Naos” (god of the spring cf. Naiads) — there was a spring below the oak in the temenos or sanctuary — and “Zeus Bouleus” (Counsellor). Originally an oracle of the Mother Goddess, the oracle was shared by Dione (whose name, like “Zeus,” simply means “deity”) and Zeus. Many dedicatory inscriptions recovered from the site mention both “Dione” and “Zeus Naios”. Elsewhere in Classical Greece, Dione was relegated to a minor role by classical times, being made into an aspect of Zeus’s more usual consort, Hera, but never at Dodona.

The god could also be invoked from a distance. In Homer’s Iliad (circa 750 BCE), Achilles prays to:

“High Zeus, Lord of Dodona, Pelasgian, living afar off, brooding over wintry Dodona”

No buildings are mentioned, and the priests (called Selloi) slept on the ground with unwashed feet. The oracle also features in Odysseus’s fictive yarn about himself told to the swineherd Eumaeus: Odysseus, he tells Eumaeus, has been seen among the Thesprotians, having gone to inquire of the oracle at Dodona whether he should return to Ithaca openly or in secret (as the disguised Odysseus is actually doing). Odysseus later repeats the same tale to Penelope, who may not yet have seen through his disguise. His words: “bespeak a familiarity with Dodona, a realization of its importance, and an understanding that it was normal to consult Zeus there on a problem of personal conduct.”

Source: Wikipedia