Dol Guldur
The Great Plague in Gondor caused the fortifications guarding Mordor to be abandoned, and Mordor again filled with evil things. The Ringwraiths took advantage of Gondor's decline to re-enter Mordor, conquered Minas Ithil, and took over the fortresses. At the time of Bilbo Baggins's quest in "The Hobbit", Sauron returned into Mordor from Dol Guldur, feigning defeat, but readying for war.