Not mentioned much in The Lord of the Rings, but a playing a crucial role in The Hobbit, is the vast expanse of forest called Mirkwood. It is the largest forest in Northwestern Middle-earth during the events of The War of the Ring and has a long and mysterious history.
It is a remnant of the ancient days of Middle-earth, being mentioned even in the earliest tales of the awakening of the first elves. Back then it was called Greenwood the Great being, of course, green and expansive. It remained this way throughout the First and Second Age of Middle-earth; many elves called Greenwood their home and many roads and realms were built within its borders. Around a thousand years into the Third Age, however, Sauron, having been defeated at the end of the Second Age, took up residence in the fortress of Dol Guldor in the Greenwood.
Soon after, the forest fell into shadow. Many evil things began to dwell there and the elves were forced into the Northeastern area of the wood. The name was changed to Mirkwood in turn and many of the roads and paths throughout the forest were lost or forgotten. In the story of The Hobbit, the company of Thorin Oakenshield must pass through Mirkwood on their quest to Erebor. They face many dangers in it such as the Enchanted River, which puts all who touch its waters to sleep; as well as abnormally large spiders.
In the same time-frame, Gandalf and the rest of the White Council venture into the fortress of Dol Guldur and banish Sauron, calling himself the Necromancer, and sending him away. He takes up refuge in Mordor and the rest of that story is covered in The Lord of the Rings. Mirkwood, though now cleansed of its main source of evil, remains dark and forbidding through the rest of the Third Age. It’s not until the ultimate defeat of Sauron at the end of that age that it returns to its former splendor. It is then renamed in the Fourth Age as the Wood of Greenleaves.