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 and very skilled a persuading people to do things that were very bad ideas.

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 at.

Character History
Warning: the following text contains spoilers.

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. At first he commanded 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 with demonic wolves. When Beren, Finrod Felagund, and Finrod's ten faithful men tried to get by him, he captured them and killed all but Beren. However, Luthien, a part-Elf, part-Maia chick, and her pet dog beat him up for capturing her boyfriend Beren, and he ran back to Morgoth, but not before concocting a version of the events that made him sound much better to tell Morgoth. 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.

Sauron decided to help heal Middle-earth. Unfortunately, he was rather impatient about it and still very flawed and, to make a long story short, he backslid. A lot. He decided to go to the Elves under the name Annatar, the Lord of Gifts (not to be confused with Santa Claus), claiming to be an emissary from Aule. 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, regardless of what Suethors may think, but secretly he made the One Ring, which coontained 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 that he was not a good guy and hid the Rings. Sauron captured Celebrimbor and tortured 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 invaded Eregion. He and the Elves fought, and when the Numenorean Men helped the Elves they drove 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 Nazgul to attack Arnor and Gondor. The Nazgul managed to capture Minas Ithil, a tower built to guard Mordor, thus winning Sauron a palantir. 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. At some point, he convinced Saruman to help him, though Saruman was still planning to take the Ring himself.

In Mordor 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. Certainly no Ringbearer would be able to make themselves destroy it, even if for some reason they decided to do such a foolish thing. 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 some so bold. The Ring would not work directly against Sauron, so he should be able to take it from Aragorn. In any case, it was entirely understandable that he didn't notice a few (very short!) trespassers playing around in his private volcano-viewing chamber. Anyway, the Ring was destroyed and he was reduced to a, to quote Gandalf, "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 look good (and a finger, obviously). His Eye is basically a symbol of his power, attention, and will, sort of like the magical, metaphorical equivalent of a Internet forum icon, not an actual physical structure. In the movie, the metaphor is, for whatever reason, made flesh flame into an 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 a woobie. This is especially strange when one considers that Suethors usually follow the movieverse, where, as mentioned above, he's a giant, flaming, disembodied Eye.