Remember you are on the Edge of the Wild now... Before you could get around Mirkwwod in the North you would be right among the slopes of the Grey Mountains, and they are simply stiff with Goblins, Hobgoblins and Orcs of the worst description. Before you could get around to the South you would get into the Land of the Necromancer; and even you Bilbo, won't need me to tell you the tales of that Black Sorcerer. I don't advise you go anywhere near the places overlooked by his Dark Tower.

The Tower of Dol Guldur

It rose like a rotten tooth out of the bowels of Mirkwood standing sentinel over the growing realm of the Dark Lord. Vast and terrible, with endless subterranean caverns below, it was still no more then a Shadow compared to the Dark Tower of Barad-dûr.

The stronghold of Sauron in the southern regions of Mirkwood, where he dwelt in secret as the Necromancer, until discovered by the White Council. When they assaulted him, he withdrew to Mordor, but the Nazgûl soon returned to reclaim Dol Guldur.

The evil Necromancer who dwelt at Dol Guldur in the heart of Mirkwood; originally thought by the Wise to be one of the Nazgûl. It was later discovered by Gandalf that the Necromancer was Sauron himself, who once again had taken shape in the world.

Khamul the Easterling - Nazgûl Lieutenant

Emissary to the peoples of the East and the Variags; Lieutenant to the Witch-King and Second among the Nine; a potent Mage in his own lifetime, now bound to the cause of Mordor forever.

He served the Dark-lord at the tower of Barad-dur during the Second-age and with his master, perished after the battle of Gorgoroth. At the end of the first millenium of the Third-age, Khamul reappeared with Sauron at the tower of Dol Guldur, and dwelt there with him while Sauron's power grew in secret. After the Dark Lord's return to Mordor, the Nazgûl Lieutenant Khamul and Adunaphel returned to Dol Guldur, where they were to hold in check the power of Lorien.