The symbolism of the Swan, the ยซHyperborean Thraceยป and the Eleusinian connection

What connection in ancient Greece linked the Hyperborean cult of Apollo, the symbolism of the swan and migratory birds, the Mysteries of Eleusis and Thracian and Eurasian shamanism? Extract from the third chapter of The Angel of the Abyss. Apollo, Avalon, the Polar Myth and the Apocalypse (Axis Mundi Editions, 2022).