Sauron

Background
In the earliest of days, before the Valar entered Arda, Sauron was a powerful Maia of Aulë the Smith, a Vala. However, Sauron was soon corrupted by the Dark Lord Morgoth ("The Great Enemy" in the tongue of men) (an evil Vala and Dark Enemy of Arda), and Sauron himself turned to evil. At first he was a spy for Morgoth, telling him the Valar's doings. Ever after, Sauron served Morgoth faithfully, and even in later days, after Morgoth was defeated and locked outside the confines of the world, Sauron encouraged and coerced some men to worship Morgoth as a god. However, while Morgoth wanted to either control or destroy the very matter of Arda itself, Sauron's desire was to dominate the minds and wills of its creatures, as well as establish himself as the ruler of Arda from his tower of Barad-dûr in Mordor. However, he originally dwelt in Angband in the Iron Mountains for a short period of time before Melkor came and claimed it again in the First Age