§ Of the Return of the Noldor The Silmarillion

Thus because of the curse that lay upon them the Noldor achieved nothing, while Morgoth hesitated, and the dread of light was new and strong upon the Orcs. But Morgoth arose from thought, and seeing the division of his foes he laughed. In the pits of Angband he caused vast smokes and vapours to be made, and they came forth from the reeking tops of the Iron Mountains, and afar off they could be seen in Mithrim, staining the bright airs in the first mornings of the world. A wind came out of the east, and bore them over Hithlum, darkening the new Sun; and they fell, and coiled about the fields and hollows, and lay upon the waters of Mithrim, drear and poisonous.