Necromancer

After Sauron's defeat at the end of the Second Age, Middle-earth had peace from evil for many centuries. After more than a thousand years of the Third Age had passed, a Shadow of darkness began to creep into the great forest known as Greenwood the Great, and it was discovered that an evil power had made a fortress for itself in the southern reaches of the forest. That fortress was Dol Guldur on the hill of Amon Lanc.

As the Shadow corrupted the Greenwood, the people of Middle-earth gave it a new name: Mirkwood. The dark being who dwelt within was also given a name, and became known as the Necromancer. However, the identity of this Necromancer was unknown even to the Wise of Middle-earth, though they suspected he might be one of the Nine Nazgûl. Eventually, the magnitude of the Necromancer's power made even the Wise fear that Sauron himself might have returned. Gandalf travelled to Dol Guldur to investigate, but the Necromancer fled into the East, leaving the Westlands in a Watchful Peace for the next four centuries.

At length, the Necromancer returned out of the East and resettled in his fortress at Dol Guldur. Once again Gandalf went there, and passing inside secrectly, he discovered that the Necromancer was indeed Sauron returned. This news convinced the White Council to move against Dol Guldur, and an assault was launched in III 2941, the same year that Bilbo located the One Ring beneath the Misty Mountains. Once again, the Necromancer gave way before the Wise, but this time there was no Watchful Peace. Instead, he reappeared a few short years later in his old land of Mordor, abandoning secrecy and revealing himself at last as Sauron the Dark Lord.