Black Shadow was an evil blight inflicted by the Nazgûl, causing their victims to slip into a dark cold sleep and never recover. Those striken by the malady murmured in their sleep until a grey shadow fell on their faces as they fell into the darkness.
After the Witch-king led the Sauron's attack on Osgiliath, news of a "Black Shadow" reached Eriador and Gandalf realized it was the sene of danger that he sense approaching.
During the Siege of Gondor many wounded and those striken by the Black Shadow were in the Houses of Healing but the healers could not find a remedy.
 See also
- ↑ J.R.R. Tolkien, The Lord of the Rings, The Fellowship of the Ring, "The Council of Elrond"
- ↑ J.R.R. Tolkien, The Lord of the Rings, The Return of the King, "The Houses of Healing"