Melkor (or Morgoth)Melkor - The original Dark Lord; the one to whom even Sauron bowed.  He was an Ainur who sought to usurp the glory of Ilúvatar.

 

"To Melkor among the Ainur had been given the greatest gifts of power and knowledge, and he had a share in all the gifts of his brethren.  He had gone often alone into the void places seeking the Imperishable Flame...But being alone he had begun to conceive thoughts of his own unlike those of his brethren." ~The Silmarillion (Ainulindalë)

 

But when the World was created by Ilúvatar and those Ainur descended upon it and became the Valar ("The Powers of the World"), Melkor desired it in his heart.

 

"Then Melkor saw what was done, and that the Valar walked on Earth as powers visible, clad in the raiment of the World, and were lovely and glorious to see, and blissful, and that the Earth was becoming as a garden for their delight, for its turmoils were subdued.  His envy grew then the greater within him; and he also took visible form, but because of his mood and the malice that burned in him that form was dark and terrible.  And he descended on Arda in power and majesty greater than any other of the Valar...Thus began the first battle of the Valar with Melkor for the dominion of Arda." ~The Silmarillion (Ainulindalë)

 

And when the Elves were awakened and suffered his torment, they gave him a new name.

 

"The mighiest of those Ainur who came into the World was in his beginning Melkor ("He who arises in Might").  But that name he has forfeited, and the Noldor, who among the Elves suffered most from his malice, will not utter it, and they name him Morgoth, the Dark Enemy of the World." ~The Silmarillion (Valaquenta)