MirkwoodWarning: Wikipedia contains spoilers
Mirkwood is a place in J. R. R. Tolkien's fictional Middle-earth. It is the forest dwelling of a elven kingdom, just east of the Misty Mountains in Rhovanion, ruled by King Thranduil. It had been called Greenwood the Great until around the year 1050 in the Second Age of the Years of the Sun when a shadow of the dark lord Sauron fell upon it and men began to call it Mirkwood. Sauron established himself at the hill-fortress of Dol Guldur, and drove Thranduil and his people ever northward, so that by the end of the Third Age they were a diminished and wary people. The Old Forest Road crosses the forest east to west, but due to its relative proximity to Dol Guldur, it is unusable. The Elves make a path farther to the north.
The great spider Shelob spawned many smaller spiders, whom Sauron introduced into Mirkwood. In The Hobbit, Bilbo and the dwarves come across these creatures. Shortly after their escape, they are captured by the Elves.
After Sauron is reduced to a powerless "spirit of malice" at the conclusion of the Third Age, the darkness lifts from Mirkwood, and it becomes known as the Wood of Greenleaves.
The name "Mirkwood" was not invented by Tolkien - it originates in old Norse and Germanic folklore, wherein it was used to refer to a number of anonymous magical forests. It has been speculated that the Black Forest of Germany derived its name from this title.