Satan

Satan — from the Hebrew śāṭān (שָׂטָן), meaning 'adversary' or 'accuser' — is the personification of evil, deception, and opposition to the divine in the Abrahamic religions. His theological character has evolved substantially: in the Hebrew Bible ha-satan is not a proper name but a functional title, describing a member of the divine court whose role is to test and accuse (most famously in the Book of Job). By the intertestamental period and firmly in the New Testament, Satan has become a distinct personal being — the fallen adversary of God, tempter of Jesus in the wilderness, and ultimate opponent of the divine plan for creation.

In Christian demonology Satan is typically identified with the serpent of Genesis 3, with the diabolos of the Gospels, and with the great red dragon of Revelation. He is often conflated (though sometimes distinguished) with Lucifer, the 'morning star' whose fall from heaven is described in Isaiah 14 and elaborated by Milton in Paradise Lost. Islamic tradition names him Iblis, a jinn who refused God's command to prostrate before the newly-created Adam and became the tempter of humankind. Judaism largely retains the older functional understanding without the personal-cosmic-adversary elaboration.

Psychologically, Satan has been read (most influentially by Carl Jung) as the personification of the shadow — the disowned, denied, or unintegrated aspects of the psyche and of the divine itself. Jungian shadow work treats encounters with satanic-feeling presences as opportunities for integration rather than events to be feared. In psychedelic and mystical reports, dark encounters with what experiencers name as Satan, the Devil, or 'demonic' presences are widely reported — often accompanied by an initially terrifying visual signature (horns, laughter, a sense of being seen through completely) that, when met without struggle, has frequently been reported to transform into an unexpected teacher-figure or into simple absence. The traditions that emphasise this figure and the traditions that avoid personifying evil at all both offer coherent approaches to the same class of experience.

Sources:
Wikipedia — Satan
Wikipedia — Shadow (psychology))