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The Unlight was a creation of the spider-like entity Ungoliant. It is described as a vast darkness that no light can escape or penetrate. As Ungoliant devoured light, she brought forth darkness.
Ungoliant wove this darkness into webs, which covered her lair in Avathar. During the Darkening of Valinor, she not only used this Unlight as a weapon, but as protection from her enemies. During the Darkening of Valinor, Ungoliant wove it as a cloak to conceal her and Melkor from the eyes of Manwë during the Darkening of Valinor. Even Tulkas, the strongest of the Valar, was unable to break through it when he pursued Melkor and Ungoliant from Valinor. When Ungoliant was denied her full reward from Melkor, she attempted to ensnare him in her Unlight. However, the webs were torn asunder by the fire of the Balrogs' whips, when they came to their masters' aid.
Though Ungoliant's descendants were incapable of producing Unlight, the webs they wove proved formidable, as they were strong enough to entrap even the mightiest warrior, and dark enough to prevent light from entering their lairs.
 Other versions of the Legendarium
In The Book of Lost Tales, the cloak that Gwerlum/Gloomweaver creates is one of invisibility rather than darkness.