At the beginning of the second millennium of the Third Age, a mysterious being came to Amon Lanc in the south of Greenwood the Great, and there built the stronghold known as Dol Guldur. This was the magician known as the Necromancer, who was later revealed to be none other than Sauron himself, and from his fortress tower a shadow of nightshade began to spread across the woodlands. From that time on, the Elves came to refer to the Greenwood as Taur-nu-Fuin, the forest under nightshade, rendered into the Mannish tongue as 'Mirkwood'.
The story of Sauron's darkening of Greenwood the Great mirrors events in the First Age, when he occupied the highland forests of Dorthonion, north of Beleriand. That region, too, fell under his shadow, and was also known as Taur-nu-Fuin.