In Tolkien's The Lord of the Rings Mirkwood was the name given to Greenwood the Great when the shadow of Dol Guldur fell on it at about TA 1050. With the growth of the power of the dark Lord Sauron in Dol Guldur this became a dark, evil place, and became over run by evil beings such as Orcs and Great Spiders. However, the forces of good still dwelt there, such as the Woodmen and the elves of the Woodland realm. By the ending of the third age the Old road that cut through Mirkwood fell into complete disuse. The elves that stayed became very secretive when the shadow passed over the wood.
When Thorin and Company passed through in TA 2941 they were led off the path by a light in the forest. When they got closer they saw they Wood elf king, and just then the lights would be kicked out, and a few minuets later the same thing would happen, until they were completely lost. It was after this that the great spiders captured them, and after they escaped that the Wood elf king, who imprisoned them, until they finally escaped, thanks to Bilbo and The One Ring, captured them. They also encountered an enchanted stream, queer eyes in the dark, insects at night, black squirrels and a great feeling of oppression and darkness.
After The War of the Ring Sauron was slain and the darkness lifted from Mirkwood, was cleansed and renamed Eryn Lasgalen.
Also called Taur e- Ndaedlos (in Sindarin), The Wood and The Great Wood.