Sauron
Those who know J.R.R. Tolkien's Middle Earth purely through The Lord of the Rings movies could be fooled into thinking that its main evil sorcerer is Saruman the White, the once pre-eminent mage who hatches an unnatural, beastly army to crush the world of men. But Sauron, that all-seeing fiery Eye, the titular Lord of the Rings, is the one who first bends Saruman's will and who, purely by being what he is, frames Tolkien's whole moral universe. Sauron, as Tolkien notes, was not always evil: he was once of the same stock as the heroic wizard Gandalf. But he lapses, gripped with that tell-tale thirst for power. The rings of power he forges to rein the peoples of Middle Earth under his sway are parables of mankind's frailty, venality and mortality. Sauron's malice may have all sorts of awesome external effects — ghoulish ringwraiths, clouds of ash, skies of dark-winged ravens — but the true extent of his evil lies in the heart of every man.