Mirkwood is a name used for two distinct fictional forests in J. R. R. Tolkien's legendarium. In the First Age, the highlands of Dorthonion north of Beleriand were known as Mirkwood after falling under Morgoth's control. During the Third Age, the large forest in Rhovanion, east of the Anduin in Middle-earth, was called Mirkwood after falling under the influence of Sauron. Before that, it was known as Greenwood the Great. 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.
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.
In The Silmarillion, the highlands of Dorthonion north of Beleriand eventually fell under Morgoth's control and was subjugated by creatures of Sauron, then Lord of Werewolves. It was renamed Taur-nu-Fuin in Sindarin, the "Forest under Deadly Nightshade". Beren who, along with Lúthien, is a foundational character of Tolkien's legendarium becomes the sole survivor of the men who once lived there as subjects of the Noldor King Finrod of Nargothrond. Beren ultimately escapes the terrible forest that even the Orcs fear to spend time in. Tolkien translated this name as Mirkwood in English. Beleg pursues the captors of Túrin through this forest in the several accounts of Túrin's tale. Along with the rest of the region west of Ered Luin, this forest disappeared after the cataclysm of the War of Wrath, although a few of its peaks may have survived as an island far off the coast of Lindon.
In the early part of the Second Age, the Woodland Realm was established in Greenwood the Great by a lord of the Sindarin Elves who had migrated eastward from Lindon. The woodland realm was a mingling of two types of elves, Sindar coming from the ruin of Doriath, and the Silvan or Wood elves who had already been settled there. Oropher, who had chosen not to depart Middle-earth after the destruction of Beleriand, chose to settle in Greenwood the Great and was taken by the Silvan Elves as their Lord. 
In The Hobbit, The Lord of the Rings, and associated writings, the expansive forest of Greenwood the Great was called Mirkwood. In this instance, the name is supposedly a translation of an unknown Westron name. Around the year 1050 of the Third Age, 'the shadow of Dol Guldur' fell upon it, and men began to call it Taur-nu-Fuin and Taur-e-Ndaedelos (Sindarin: forest of great fear). The shadow was the power of Sauron who, under a concealed identity, established himself at the hill-fortress of Dol Guldur on Amon Lanc. The presence of Sauron's minions drove the Elves (now led by Thranduil, son of Oropher) further northward, so that by the end of the Third Age they were a diminished and wary people who had entrenched themselves beyond the Mountains of Mirkwood (Emyn Fuin, formerly the Emyn Duir or "Dark Mountains"). The Old Forest Road or Old Dwarf Road crossed the forest east to west, but due to its relative proximity to Dol Guldur, the road was mostly unusable. The Elves made a path farther to the north, which ended somewhere in the marshes south of the Long Lake of Esgaroth.
In The Hobbit, Bilbo Baggins, along with Thorin Oakenshield and his band of Dwarves — minus the Wizard, Gandalf — ventured into Mirkwood during their quest to regain Erebor from the Dragon Smaug. During their passage through Mirkwood, the party encountered and was captured by many Giant Spiders, descended from Shelob. Shortly after the dwarves' escape they were taken prisoner by the Elves and brought before Thranduil, who imprisoned the dwarves. While unclear, it was shortly after or possibly even during these events, that the White Council flushed Sauron out of Dol Guldur, and as he fled to Mordor his influence in Mirkwood diminished for a while.
Years later, Gollum, after his release from Mordor, was captured by Aragorn and brought as a prisoner to Thranduil's realm. Out of pity, they allowed the creature some freedom to roam the forest (under close guard). Gollum escaped custody during an Orc raid, and fled south to Moria in search of the One Ring.
After Sauron's destruction at the conclusion of the Third Age, Mirkwood was cleansed by Galadriel and became known as Eryn Lasgalen, Sindarin for the Wood of Greenleaves.
Geography and Climate
Mirkwood was located in Rhovanion, east of the Anduin in Middle-earth. It lies east of the Misty Mountains' rain shadow and has a humid-continental climate; winters are cold throughout but much longer in the north, while the south has hotter summers.
Mirkwood contained one of the few remaining Elven settlements of the Fourth Age. It was inhabited by Silvan elves, who were more reluctant to depart Middle-earth than their Noldorin kin. Those from Lothlórien who did not accompany Galadriel to the West migrated to the forest during the early Fourth Age.
Notes and references
- King Sheave, The Lost Road and Other Writings, 91 
- New York Times Book Review, The Hobbit, by Anne T. Eaton, March 13, 1938, "After the dwarves and Bilbo have passed ...over the Misty Mountains and through forests that suggest those of William Morris's prose romances." (emphasis added)
- Lobdell, Jared . A Tolkien Compass. La Salle, IL: Open Court. ISBN 0-87548-316-X. p. 84, "only look at The Lord of the Rings for the briefest of times to catch a vision of ancient forests, of trees like men walking, of leaves and sunlight, and of deep shadows."
- The headstone above the grave containing J.R.R. Tolkien and his wife, Edith, also names him 'Beren' and her 'Lúthien - carved in stone.
- J.R.R. Tolkien, The Lays of Beleriand, Houghton Mifflin, 1985, p. 36, "but dread they know of the Deadly Nightshade and in haste only do they hie that way."
- Unfinished Tales: "The History of Galadriel and Celeborn - Appendix B: The Sindarin Princes of the Silvan Elves," p. 258-59; "The Disaster of the Gladden Fields," p. 280-81 note 14
- The History of Middle-earth, vol. V, The Lost Road and Other Writings: "The Etymologies," ORO and PHER
- Robert Foster, A Guide to Middle Earth, NY:Ballantine Books (Random House), 1971, p. 251
- Robert Foster, A Guide to Middle Earth, NY:Ballantine Books (Random House), 1971, p. 174
- Evans, Jonathan (2006). "Mirkwood". In Drout, Michael D. C.. J. R. R. Tolkien Encyclopedia: Scholarship and Critical Assessment. Routledge. pp. 429–430. ISBN 0-415-96942-5.
- at the
- Tolkien in the land of Arthur: the Old Forest episode from The Lord of the Rings. Mythopoeic Society, 2006. An article discussing the significance of forests in Tolkien's work, in particular, the Old Forest with comparisons to other myths and romances.
- Mirkwood at the Encyclopedia of Arda