In the same work, he is revealed to be the same character as “the Necromancer” from Tolkien’s earlier novel The Hobbit. In Tolkien’s The Silmarillion (published posthumously by Tolkien’s son Christopher Tolkien), he is also revealed to have been the chief lieutenant of the first Dark Lord, Morgoth. Tolkien noted that the “angelic” powers of his constructed myth “were capable of many degrees of error and failing”, but by far the worst was “the absolute Satanic rebellion and evil of Morgoth and his satellite Sauron.”
Around the year 1050, a shadow of fear fell on the forest later called Mirkwood. As would later become known, this was the first intimation of Sauron manifesting himself yet again. He established a stronghold called Dol Guldur, “Hill of Sorcery”, in the southern part of the forest. In Mirkwood he was known as the Necromancer (mentioned briefly in The Hobbit), but the Elves did not recognize him at first.
The Valar would not act to defeat Sauron in a massive intervention comparable to the War of Wrath that overthrew Morgoth; rather, they sent five Maiar in the form of Wizards, the most prominent being Gandalf and Saruman.
Around the year 1100, “the Wise” (the Wizards and the chief Elves) became aware that an evil power had made a stronghold at Dol Guldur. Initially it was assumed that this was one of the Nazgûl rather than Sauron himself. About the year 1300, the Nazgûl did indeed reappear, and their influence would have serious consequences for the nations established by the Númenórean exiles.
Over the ensuing centuries, the Witch-king of Angmar (the chief Nazgûl, acting on Sauron’s behalf) repeatedly attacked the northern realm of Arnor, first in 1409 and finally overrunning the realm in 1974. Six years later, the Witch-king entered Mordor and gathered the Nazgûl there. In 2000, the Nazgûl issued from Mordor and took the city of Minas Ithil (later known as Minas Morgul) in one of the mountain-passes. Thereby they also captured an object that would prove most valuable to Sauron: a palantír, one of the seven seeing stones that Elendil’s people had brought with them from Númenor at the eve of the Downfall.
As the power of Dol Guldur grew, the Wise came to suspect that the controlling force behind the Witch-king and the other Nazgûl was indeed their original master, Sauron. In 2063, Gandalf went to Dol Guldur and made the first attempt to learn the truth, but Sauron retreated and hid in the East. It would be almost 400 years before he returned to his stronghold in Mirkwood, and his identity remained undetermined.
Sauron finally resurfaced with increased strength in 2460. About the same time, the long-lost Ruling Ring was finally recovered from the River Anduin, found by a hobbit named Déagol. His relative Sméagol killed him for the Ring, and was eventually corrupted into the creature Gollum. Banished by his family, he took the Ring, which he called his “Precious,” and hid in the Misty Mountains.
In 2850, Gandalf made a second attempt to spy out Dol Guldur. Stealing into the stronghold, he was finally able to confirm the identity of its lord, later reporting to the White Council of Elves and Wizards that Sauron had returned. Saruman, hoping thereby to acquire the One Ring for himself, dissuaded the Council from acting against him.
Eventually, the Wizards and chief Elves combined to put forth their might, and drove Sauron out of Mirkwood in 2941. During the White Council’s delay he had, however, prepared his next move, and was willing to abandon Dol Guldur.
Just before Sauron fled Dol Guldur, the hobbit Bilbo Baggins, on an improbable adventure with a party of Dwarves, stumbled across the Ring deep within the Misty Mountains. After his quest was over, Bilbo brought the Ring back to Hobbiton in the Shire. Decades later, he passed it on to his heir, Frodo.
Sauron’s power had now recovered to the point that he was able to extend his will over Middle-earth. The Eye of Sauron, as his attention and force of will was perceived, became a symbol of oppression and fear. Following his expulsion from Dol Guldur, he returned to Mordor in 2942, openly declared himself nine years later, and started raising Barad-dûr anew. In preparation for a final war against Men and Elves, he bred immense armies of Orcs, augmenting them with Men from the East and South to create the monstrous Uruk-hai.