Sauron was the greatest enemy of the free peoples of Middle-earth in the Second and Third Ages. He was once a servant of Morgoth, but after Morgoth's defeat, Sauron became a Dark Lord himself and sought to conquer all of Middle-earth. He forged the One Ring to exert his will over others, but in the end it was the instrument of his downfall. 

Sauron was one of the Maiar - spirits who helped and served the Powers known as the Valar. He came into existence before the creation of the world. Sauron was not evil in the beginning. At first he was a Maia of Aule, the Vala whose domain was the substances of which the earth was made. Aule was a smith and a master of crafts and works of skill, and Sauron learned much from him about making, forging, and creating. 

But Sauron was drawn to the strength and might of Melkor - a Power who desired dominion over the entire world. Sauron craved order and wanted to arrange things according to his will, and he believed that following Melkor was the best way to achieve this end. Melkor became known as Morgoth - the Dark Enemy - and Sauron became his most powerful minion and performed many evil deeds in his service. 

