Dol Guldur
After Sauron was defeated in the War of the Last Alliance, he retreated to Amon Lanc and built a fortress, where he hid in secrecy while recovering his strength. As the evil force residing there became apparent to the outside world, it was named "the Necromancer". It was not known at first that Sauron had regained physical form and power; it was believed that a "Nazgûl" dominated the tower and land. But as the power grew in strength, Gandalf the Grey became suspicious and travelled to Dol Guldur to investigate. Sauron anticipated his coming and withdrew to the East to remain hidden in secrecy. Thus began what later became known as The Watchful Peace, which lasted until Sauron returned to Dol Guldur after years of hiding. The Wise (various Elf-lords and Wizards, including Gandalf the Grey) then formed the White Council to determine whether Sauron was alive and regaining strength.