Dol Guldur
The fictional history of Dol Guldur is laid out across several of Tolkien's works, including "The Lord of the Rings", "The Hobbit", "The Silmarillion", and "Unfinished Tales". Sauron's history at the site begins in the Third Age, around the year 1100. Due to its situation inside Mirkwood and east of Lothlórien, Dol Guldur was a key stronghold in Sauron's return to power.