Woodland Realm

Mirkwood is a name used for two distinct fictional forests on the continent of Middle-earth in J. R. R. Tolkien's legendarium.

One of these occurred in the First Age of Middle-earth, when the highlands of Dorthonion north of Beleriand became known as Mirkwood after falling under Morgoth's control.

The other Mirkwood, and the more famous of the two, was the large forest in Wilderland, east of the Anduin. It had acquired the name Mirkwood during the Third Age, after it fell under the influence of the Necromancer; before that it had been known as Greenwood the Great. This Mirkwood features significantly in The Hobbit and in the film The Hobbit: The Desolation of Smaug.

The term Mirkwood is taken from William Morris, influenced by the forest Myrkviðr of Norse mythology. Projected into Old English, it appears as Myrcwudu in Tolkien's The Lost Road, as a poem sung by Ælfwine. Tolkien also used the term Mirkwood in another unfinished work, The Fall of Arthur.

Forests play an enormous role throughout the invented history of Tolkien's Middle-earth and are inevitably an important episode on the heroic quests of his characters. The forest device is used as a mysterious transition from one part of the story to another.

...
Wikipedia