The Silmarillion

Now Fingolfin, King of the North, and High King of the Noldor, seeing that his people were become numerous and strong, and that the Men allied to them were many and valiant, pondered once more an assault upon Angband; for he knew that they lived in danger while the circle of the siege was incomplete, and Morgoth was free to labour in his deep mines, devising what evils none could foretell ere he should reveal them. This counsel was wise according to the measure of his knowledge; for the Noldor did not yet comprehend the fullness of the power of Morgoth, nor understand that their unaided war upon him was without final hope, whether they hasted or delayed. But because the land was fair and their kingdoms wide, most of the Noldor were content with things as they were, trusting them to last, and slow to begin an assault in which many must surely perish were it in victory or in defeat Therefore they were little disposed to hearken to Fingolfin, and the sons of Fëanor at that time least of all. Among the chieftains of the Noldor Angrod and Aegnor alone were of like mind with the King; for they dwelt in regions whence Thangorodrim could be descried, and the threat of Morgoth was present to their thought. Thus the designs of Fingolfin came to naught, and the land had peace yet for a while.

But when the sixth generation of Men after Bëor and Marach were not yet come to full manhood, it being then four hundred years and five and fifty since the coming of Fingolfin, the evil befell that he had long dreaded, and yet more dire and sudden than his darkest fear. For Morgoth had long prepared his force in secret, while ever the malice of his heart grew greater, and his hatred of the Noldor more bitter; and he desired not only to end his foes but to destroy also and defile the lands that they had taken and made fair. And it is said that his hate overcame his counsel, so that if he had but endured to wait longer, until his designs were full, then the Noldor would have perished utterly. But on his part he esteemed too lightly the valour of the Elves, and of Men he took yet no account.

There came a time of winter, when night was dark and without moon; and the wide plain of Ard-galen stretched dim beneath the cold stars, from the hill-forts of the Noldor to the feet of Thangorodrim. The watch-fires burned low, and the guards were few; on the plain few were waking in the camps of the horsemen of Hithlum. Then suddenly Morgoth sent forth great rivers of flame that ran down swifter than Balrogs from Thangorodrim, and poured over all the plain; and the Mountains of Iron belched forth fires of many poisonous hues, and the fume of them stank upon the air, and was deadly. Thus Ard-galen perished, and fire devoured its grasses; and it became a burned and desolate waste, full of a choking dust, barren and lifeless. Thereafter its name was changed, and it was called Anfauglith, the Gasping Dust. Many charred bones had there their roofless grave; for many of the Noldor perished in that burning, who were caught by the running flame and could not fly to the hills. The heights of Dorthonion and Ered Wethrin held back the fiery torrents, but their woods upon the slopes that looked towards Angband were all kindled, and the smoke wrought confusion among the defenders. Thus began the fourth of the great battles, Dagor Bragollach, the Battle of Sudden Flame.

In the front of that fire came Glaurung the golden, father of dragons, in his full might; and in his train were Balrogs, and behind them came the black armies of the Orcs in multitudes such as the Noldor had never before seen or imagined. And they assaulted the fortresses of the Noldor, and broke the leaguer about Angband, and slew wherever they found them the Noldor and their allies. Grey-elves and Men. Many of the stoutest of the foes of Morgoth were destroyed in the first days of that war, bewildered and dispersed and unable to muster their strength. War ceased not wholly ever again in Beleriand; but the Battle of Sudden Flame is held to have ended with the coming of spring, when the onslaught of Morgoth grew less.

Thus ended the Siege of Angband; and the foes of Morgoth were scattered and sundered one from another. The most part of the Grey-elves fled south and forsook the northern war; many were received into Doriath, and the kingdom and strength of Thingol grew greater in that time, for the power of Melian the queen was woven about his borders and evil could not yet enter that hidden realm. Others took refuge in the fortresses by the sea, and in Nargothrond; and some fled the land and hid themselves in Ossiriand, or passing the mountains wandered homeless in the wild. And rumour of the war and the breaking of the siege reached the ears of Men in the east of Middle-earth.