Mirkwood is a large forest to the east of the Misty Mountains. At the beginning of the Third Age, its name was changed from Greenwood the Great to Mirkwood to reflect its dark, sinister nature. This was caused by the building of the evil fortress Dol Guldur (‘Hill of Sorcery’) by a mysterious necromancer. This evil power was driven out once, fleeing eastwards, but returned to Dol Guldur approximately four hundred years later. Gandalf investigated and found that the necromancer was none other than Sauron, gathering his power again after his defeat at the end of the Second Age. The White Council, a gathering of the wizards and the Chief Eldar, met to discuss the threat and eventually agreed to drive Sauron out. By now, however, it was too late, and Sauron only fled as far as Mordor to begin gathering his armies again.
Tolkien created a rich history and mythology of Middle-earth that spans many ages. There are numerous references to these events and stories throughout the book, and a collection of more detailed accounts in a large appendix at the end of The Return of the King, which can sometimes be a little overwhelming. More about Middle-earth can also be found in The Silmarillion, Unfinished Tales, and The History of Middle-earth, all edited and published after Tolkien’s death by his son, Christopher Tolkien.
In order to make sense of all this information and background, there are many unofficial companion books and encyclopaedias written for The Lord of the Rings, with entries on all the people, places and battles mentioned. These can be very useful for those interested in delving deeper into Middle-earth’s past as they read the story. Alternatively, plenty of information can be found online (Warning: these links include Spoilers!):