Sauron - A lieutenant of Morgoth in the First Age, Sauron rose to become the Dark Lord after his master was banished by the Valar.  He attempted to bring all the free peoples of Middle Earth under his control by forging the Ring of Power in the Second Age, but was ultimately defeated when the ring was destroyed at the end of the Third Age.

"It seemed that the evil power of Mirkwood had been driven out by the White Council only to reappear in greater strength in the old strongholds of Mordor.  The Dark Tower had been rebuilt, it was said.  From there the power was spreading far and wide, and away far east and south there were wars and growing fear.  Orcs were multiplying again in the mountains.  Trolls were abroad, no longer dull-witted, but cunning and armed with dreadful weapons...Sauron the Great, the Dark Lord...he has indeed arisen again and left his hold in Mirkwood and returned to his ancient fastness in the Dark Tower of Mordor." ~The Fellowship of the Ring, Book I, chap. 2 (The Shadow of the Past)