Name used in the Third Age for the great forest of Rhovanion east of the River Anduin, formerly Greenwood the Great and later Eryn Lasgalen.

The name ‘Mirkwood’ was only used after about TA 1050 when Sauron (then called the Necromancer) was found to be living in Dol Guldur. Although Sauron fled to Mordor when discovered, he later stationed some of the Nazgul there and as evil spread throughout the wood, the elves named it Taur e-Ndaedelos (Forest of Great Fear).

Mirkwood was traversed by the old Old Forest Road and the Elf Path, both somewhat neglected by the end of the Third Age, the latter being infested with huge malicious Spiders. In the north the Elvenking, Thranduil protected a kingdom around the Forest River. Further south around the Old Forest Road there lived Wood-men and Beornings. The wizard Radagast the Brown also made his home there.

The Mountains of Mirkwood formed a small group at the heart of the forest from which flowed streams of dangerously enchanted water, one of which crossed the Elf Path. The realm of Lothlorien occupied the western bank of the Anduin, opposite the southern part of the forest and was protected from its evil by the ring Nenya kept by the Lady Galadriel.

After the destruction of Sauron in TA3019 Dol Guldur was cast down and the forest renamed Eryn Lasgalen (Wood of Green Leaves by Thranduil and Celeborn, the latter taking up residence there in a region named East Lorien.
Encyclopedia entry originally written by Hathaldir