Sauron

Sauron is the Big Bad of The Lord of the Rings. In fact, he is the Lord of the Rings. He is also known as Annatar, Aulendil, Gorthaur, Mairon, Thu, the Lord of Wolves, and a lot of other names and titles. He first was mentioned in published material in The Hobbit, where he is called "The Necromancer". His role in The Silmarillion was originally taken by a giant cat named Tevildo. He is an extremely powerful being who is very skilled at persuading people to do things that are bad for both themselves, and the world.

He teaches at the Official Fanfiction University of Middle-earth and has a rivalry with his ex-boss Morgoth, in whose direction he is often seen flicking peanut shells and the like.

Character History
Sauron was originally a Maia who served Aule, the Vala of craftsmanship. However, he left and became one of Melkor's chief servants. At first he acted as a spy, but he later openly served Melkor. He was the first commander of Angband, but after Utumno was destroyed Melkor took it as his main fortress. Later in the Dagor Bragollach, he captured the Elves' fortress of Minas Tirith (not to be confused with the later one) and occupied it with demonic wolves.

When Beren, Finrod Felagund, and their company tried to get by him, he captured them and killed all but Beren. However, he was defeated by Luthien and Huan, and only his talent at spin enabled him to survive Morgoth's wrath. After Morgoth was defeated by the Valar, Sauron repented, but he was unwilling to undergo the humiliation of going back to Valinor. Instead, he hid in Middle-earth, which he regarded as abandoned by the Valar.

Sauron decided to help heal the continent, but his impatience and lingering desire for domination caused him to backslide. After gathering power among the remnants of Morgoth's forces in the East, he decided to go to the Elves and pose as an emissary from Aule under the name Annatar, Lord of Gifts. Gil-Galad, Elrond, and Galadriel mistrusted him, but many of the smiths of Eregion gladly worked with him. One of them was Feanor's grandson, Celebrimbor. With them he made sixteen of the Rings of Power, but secretly he made the One Ring, which contained much of his power.

The Elves also made three Rings without him, the last and greatest. When Sauron put on the One Ring, the Elves realized his true intentions and hid the Rings. This caused Sauron to invade Eriador, capturing Celebrimbor and torturing him in an attempt to find out where the Rings, particularly the last three, were; Celebrimbor told him where the other Rings, which Sauron had already helped make and tainted, were, but refused to tell him where the Three were. Angered, Sauron killed him and used his body as a flag when he pressed on towards Lindon. Thankfully, the Numenorean Men helped the Elves drive him away, but Eregion was destroyed.

Later the Numenoreans, led by their king Ar-Pharazon, returned to destroy him. Sauron's armies deserted, and he surrendered to Ar-Pharazon and convinced the king to bring him back to Numenor as a prisoner. Before too long, he had gone from being a prisoner to being Ar-Pharazon's most trusted adviser and chief priest in a Morgoth-worshiping cult. Then he convinced Ar-Pharazon to attack the Valar, and Eru Iluvatar responded by destroying Numenor and making Earth round.

After that, Sauron re-established himself in Mordor and fought with the Last Alliance between the Elves and the few living, non-Morgothist Numenoreans. Sauron killed Gil-galad and Elendil, but was defeated by Isildur who, not knowing the Ring's precise nature, took it as a weregild for his father.

Greatly weakened, Sauron moved to what became Dol Guldur, a fortress near Mirkwood. There he worked to recover some of his former power and became known as The Necromancer. The Elves and Wizards knew that something evil lived there, but suspected - or hoped - it was a Nazgul. Meanwhile, Sauron sent the actual Ringwraiths to attack Arnor and Gondor. These managed to capture Minas Ithil, thus winning Sauron a palantir.

Later, Gandalf managed to work his way into Dol Guldur, realized it was Sauron, and reported back to the Elves and other Wizards. They - after Saruman stalled them for a while - attacked Dol Guldur, and Sauron left for Mordor. When Saruman acquired the land of Isengard, Sauron managed to use the palantir there to convince Saruman to join his side, though the Wizard was still planning to take the Ring himself. Meanwhile, Sauron rebuilt his armies.

From Gollum he learned that the One Ring was held by "Baggins" from the "Shire", and he sent the Nazgul to find the Ring. Sauron believed that, logically, his enemies would unite under one strong leader who used the Ring. From his point of view, no Ringbearer would be able to make themselves destroy it, even if for some reason they decided to do so. Aragorn was the obvious choice of Ringbearer, so when the united armies of Gondor and Rohan marched on the Black Gate of Mordor, Sauron figured that only someone who had the Ring would do something so bold.

This caused him to be distracted, allowing Frodo and Sam to destroy the Ring. This caused him to be destroyed, reducing him to a "mere spirit of malice that gnaws itself in the shadows, but cannot again grow or take shape."

An important note about the Eye of Sauron: In the books, Sauron has a humanoid shape, except when he chooses to shift it. During the events of The Lord of the Rings, he looks like a terrible dark lord with four fingers on one hand, having lost the ability to shift into a fair form. His Eye serves a symbol of his power, attention, and will, not an actual physical form (though this is debated). In the movie, the metaphor is, for whatever reason, made into a giant, flaming, disembodied Eye atop Barad-dur. This is apparently his physical(ish) form in the movieverse.

In Badfic
Many Mary Sues claim to be his Daughter. Other Mary Sues reform him with the power of Love or turn evil. Sometimes he's even Woobiefied. This is despite the fact that, as mentioned above, he is a giant, flaming, disembodied Eye (at least in the movieverse, which is likely the only Lord of the Rings universe such Sues are familiar with.)