Sauron

Sauron is the manifest visage of all evil in the World. As the Dark Lord, he seeks to establish his dominion over Middle-earth by regaining his One Ring and conquering the lands of the Free Peoples through his servants: Saruman and the Nazgûl.

The Forging of the Rings
In the Second Age, Sauron helped forge the Rings of Power: three for the Elves, seven for the Dwarves and nine for Men. In secret, he made the One Ring, to control all others and enslave all life on Middle-earth.

With his Ring on his finger he was unstoppable, until during the War of the Last Alliance, he lost it to Prince Isildur and was believed to have been defeated for the last time.

Return
In the Third Age, the wizard Gandalf the Grey found out the truth that Sauron had in fact survived and was preparing for war.

He had sent the Nazgûl led by the Witch-king to find the hobbit Frodo Baggins who had the Ring with him, while also having gained the allegiance of the wizard Saruman.

With his servants ready, the Dark Lord started the hunt for the Fellowship of the Ring, trying numerous time to stop them. He had managed to send his Orcs in Moria and had some alliance with the Balrog living there.

Trivia

 * Sauron appears phisically only in the prologue with the forging of the Rings where he appears as a tall shadowy figure filled with malice and power.
 * He is rather the secondary antagonist as Saruman has more screen time and is shown to be more of a threat to the Fellowship of the Ring than Sauron.