Socrates had a daimon, and it was nothing like a demon
To the ancient Greeks a daimon was simply a spirit or divine power; Plato used the word for the inner voice that guided Socrates. Early Christians recast those neutral spirits as agents of evil, even seeing them lurking inside old pagan statues. The demon we picture today is the product of that switch.
Demons appear in folklore, religion, magic and fiction worldwide, and belief in them probably reaches back to the Palaeolithic. Anthropologists link them to fear of the unknown: an unexplained death, a mysterious disease or a hostile stranger gets turned into a thinking agent with bad intentions. The same logic can demonise outsiders, and people expelled from a community may be imagined returning as malevolent presences.
The Greek word daimon probably comes from a verb meaning to divide or allot, and it sat close to the Latin genius or numen. By the early Roman Empire, pagans and Christians alike felt that cult statues were inhabited by divine power. Christians simply reclassified those presences as Satan's followers, and well into the Byzantine era they regarded old civic sculptures as haunted. English was already using demon to mean devil by around 825, while German still separates Dämon from Teufel.
Older civilisations had their own schemes. Egyptian had no exact word for demon, but magical texts wrote malevolent spirits' names in red ink. Scholars sort Egyptian demons into guardians, tied to places such as the gates of the underworld and dangerous only to those without the right spells, and wanderers, bringers of plague, madness and nightmares, which magic could keep at bay but never destroy. In the Babylonian creation epic Enūma Eliš, gods and demons alike are offspring of Tiamat, goddess of primordial chaos; Mesopotamians wore amulets against spirits who carried storms and headaches.
Zoroastrianism shaped the later Abrahamic picture. By raising Ahura Mazda to sole supreme god, Zoroaster demoted the daevas to demons and taught that they turned evil by their own choice; later tradition made Ahriman their leader. In modern psychology the word survives as metaphor, in the inner demons people wrestle with.
Source: Demon
