Sauron (aka Mairon, Annatar, Sauron the Great, Gorthaur the Cruel, Sauron the Fair, The Enemy, The Dark Lord, the Eye) is the Big Bad Evil Guy of the Lord of the Rings franchise and the reason behind much of the changes and upheaval that the LoR world has faced during it's current ages. He is one of the first proper Dark Lords in popular fiction; while Darth Sidious, Doctor Doom and Abaddon were in kindergarten like some evil version of the muppet babies learning what the word 'bad' meant, Sauron was out cracking heads and getting shit done.
He wasn't the original big bad for LoR, oh no
A hilarious note is the fact he wasn't even the original big bad to begin with; Morgoth the first
Dark Lord Giver of Freedom seduced Sauron (he was called Mairon back then) into his new lifestyle by offering him unlimited chocolate cake forever and Sauron agreed, quickly becoming his chef chief underling. This was quite perplexing to those who originally knew Mairon, as he was a lover of order and organization where Melkor just liked to fuck shit up; but Melkor got shit done where all the others did basically jack shit. Queue a long string of fucking shit up along with his new boss, until he ran into a girl and her dog (read: an OP elven princess and the hound of the local equivalent of the god of the hunt), and got himself almost killed. Once his boss was beaten up by the other Ainur (Tolkien's versions of archangels in this universe), Sauron fled across the sea to Middle-Earth, where he used his great cunning and sorcery to work events to his favour.
At first, he tried to corrupt the Elves by 'helping' develop their magic. When this didn't work out, he corrupted the Numenorians (the LotR equivalent of Atlantis) and caused so much harm the other Maia used their full power to defeat Sauron and sunk Numenor; an action which made their ruler Eru (the LotR equivalent of the Judeo-Christian God) forbid them from using their full power so as to reduce collateral damage.
Damn those pesky kids!
Like any true evil villain, Sauron was then beaten time and time again but he always managed to flee while screaming "I'll be back" over his shoulder. Finally having had enough of subtlety, he tried to merely beat everyone to death in his big destroyer form with an army of orcs but thanks to some pesky men and elves, he got his finger with his magic ring (which contained a great deal of his power and life) cut off and as a result became a highly pissed off spirit that kept trying to come back, knowing that if he could get his bling back again he could have another bash at taking over the world.
Rise and fall again, this time for good
Sauron managed to crawl (do spirits crawl?) his way back from the abyss through sheer stubbornness and over time rebuilt his strength. When he felt secure enough he returned to his dark kingdom Mordor and began preparing for his final war against the kingdoms of men and elf, breeding an army of Uruk-Hai (think Primaris Marines with a whole lot more evil) to do his bidding. But you remember that bling we mentioned earlier? Some dang pesky hobbit kid only went and dropped it in Mount Doom! The one volcano that could destroy it! You know what we are saying?! Yep, it was bye bye for good this time Sauron and the one guy who made the world interesting popped his cogs.
And everyone went home in time for
If you want to watch him work
Of course, there's the obvious prologue scene in the first LOTR film, but you can also have him as a hero unit in the Battle for Middle-Earth 2 videogame, if you play any of the 3 Evil factions. Be warned, you have to find and kill Gollum (and he is a BITCH to find) just to get his bling to be able to summon him. Then you have to pay a LARGE amount of your resources and wait for him to turn up. But seriously, for a guy who can smash apart entire squads IN ONE HIT, call down a hail of molten rock and scare anything attacking him into running away? He's worth it. Every second you wait to get him, he will pay back in full. (Just don't drop him in the middle of the enemy, dumbass.) And the best bit? All that shit is fucking CANON.