Melkor, later known as Morgoth, the Dark Lord of the First Age
Dark Lord was the title of the two most powerful evil overlords in Arda, each of whom worked to take control of all Middle-earth for themselves. That which they could not control, they sought to destroy. The first and greater Dark Lord was the Vala first known as Melkor and then as Morgoth. The second and lesser Dark Lord was the Maia, and former lieutenant of Morgoth known as Sauron. Both Dark Lords were very influential in the history of Middle-earth, waging numerous wars against the Valar, the Elves, and a few select groups of Men. Their actions, especially those of Morgoth, had a permanent negative impact on the world, as the Dark Lords both destroyed many beautiful things and places, and caused untold anguish and grief. Eventually both Dark Lords were overthrown and defeated; Melkor was cast into the Void at the end of the First Age by his brethren, and Sauron was permanently defeated at the end of the Third Age by Frodo Baggins with the aid of Samwise Gamgee and Gollum, when they cast the One Ring into Mount Doom and robbed Sauron of most of his power.
Behind the Scenes
"Dark Lords" became something of a cliché in fantasy and science fiction novels and other media following the success of J.R.R. Tolkien's The Lord of the Rings with the Dark Lord Sauron.
Later in the Silmarillion it is revealed that Sauron was the second Dark Lord and that the first was Morgoth with Sauron actually being a servant of Morgoth. When Morgoth fell, Sauron took up his throne of influence and vowed to complete what his master initiated.