"And darkness [Melkor] used most in his evil works upon Arda, and filled it with fear for all living things." —The Valaquenta, "Of the Enemies"
Darkness was a major weapon that the Dark Lords of Middle-earth, Melkor-Morgoth and Sauron, used to instill fear in the Free peoples that stood against them. Sometimes they used Darkness as a frightening concept to coax people to rely on them for protection, while at other times the Dark Lords utilized Darkness as an actual force. This article relates time periods in which the rebellious Powers spread literal Darkness over places on Earth.
The Great Darkness
The "Great Darkness" was an uncertain term used by Treebeard the Ent, apparently referring to the time that Middle-earth was under the dominion of Morgoth. It was in the Great Darkness before the Sun and Moon that Melkor first created the Orcs and the Trolls, so that these creatures feared sunlight and shunned it. The Darkness described by Treebeard was not dispelled by the coming of the Sun: Morgoth held his fortress of Angband for centuries afterwards, and kept it shrouded in darkness with vapors belched forth from Thangorodrim, the mountains around Angband.
Morgoth, aided by the power of Ungoliant the Spider, also used Darkness to devastating effect in his attack on the Two Trees of Valinor. After Ungoliant drank up their light, a debilitating darkness settled around the Blessed Realm that confounded the might of even most of the Valar. Aided by this force, which was more than merely a lack of light but also "had power to pierce the eye, and to enter heart and mind, and strangle the very will", Morgoth escaped from Valinor and revenged himself upon the Valar and Elves. Only the sharp eyes of Manwë could penetrate this shroud, but it was to no avail: Morgoth used the cover of Darkness to slay Finwë and capture the Silmarils of Fëanor.
During the Third Age
"The Darkness" was a term usually used symbolically for the dark power of Sauron and the regions under his sway. During the War of the Ring, though, Sauron's Darkness became a dreadful reality, as it spread out of Mordor and covered the lands of the West for many days. It caused despair among the soldier of Gondor who were fighting Sauron's forces. Eventually, however, a wind from the South broke it apart.
The Darkness and Night
Ideally, Night was supposed to be a peaceful time, no less threatening than Day. The Valar had intended it as period of rest and repose, in which one could view the stars of Varda. But the association of Night with Darkness and Morgoth's power caused it to become "a time of peril unseen, of fear without form, an uneasy vigil; or a haunted dream, leading through despair to the shadow of Death". These connotations were especially strong in Middle-earth; in safer lands such as Aman the Children of Ilúvatar could still enjoy Night as originally intended.