Black Shadow
From Tolkien Gateway
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.[1]
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.[2]
See also