Several distinct figures were merged into the Christian Devil, and untangling them is most of what can usefully be said.
Diabolos is the Greek word the Septuagint uses to render satan, and it means slanderer. It becomes a name in Greek and Latin Christianity, giving the English devil. Beelzebul in the Gospels is a title of the prince of demons, derived from a Philistine god at Ekron whose name appears in the Hebrew Bible in a deliberately insulting altered form, lord of the flies. Belial is a separate term from the Hebrew Bible, meaning worthlessness, personified in the Dead Sea Scrolls. Lucifer is an entirely different accident, treated in its own entry.
The fall of Satan is assembled from passages that are not about him. Isaiah 14 is a taunt against a king of Babylon. Ezekiel 28 is a lament over the king of Tyre. Luke has Jesus say he saw Satan fall like lightning, without a narrative. Revelation 12 describes a war in heaven and the dragon thrown down, and identifies the dragon with the ancient serpent, Satan, which is the source of the identification with the serpent of Eden. Genesis 3 does not name Satan. It has a serpent, described as the most cunning of the animals God made.
The elaborate biography, pride before creation, a third of the angels, the war, the pit, is patristic and medieval theology, given its dominant English literary shape by Milton in 1667. Paradise Lost is not a source. It is a poem, and it has done more than any doctrine to fix the picture.