The Silmarillion

Thus it was that when Nahar neighed and Oromë indeed came among them, some of the Quendi hid themselves, and some fled and were lost. But those that had courage, and stayed, perceived swiftly that the Great Rider was no shape out of darkness; for the light of Aman was in his face, and all the noblest of the Elves were drawn towards it.

But of those unhappy ones who were ensnared by Melkor little is known of a certainty. For who of the living has descended into the pits of Utumno, or has explored the darkness of the counsels of Melkor? Yet this is held true by the wise of Eressëa, that all those of the Quendi who came into the hands of Melkor, ere Utumno was broken, were put there in prison, and by slow arts of cruelty were corrupted and enslaved; and thus did Melkor breed the hideous race of the Orcs in envy and mockery of the Elves, of whom they were afterwards the bitterest foes. For the Orcs had life and multiplied after the manner of the Children of Ilúvatar; and naught that had life of its own, nor the semblance of life, could ever Melkor make since his rebellion in the Ainulindalë before the Beginning: so say the wise. And deep in their dark hearts the Orcs loathed the Master whom they served in fear, the maker only of their misery. This it may be was the vilest deed of Melkor, and the most hateful to Ilúvatar.

Oromë tarried a while among the Quendi, and then swiftly he rode back over land and sea to Valinor and brought the tidings to Valmar; and he spoke of the shadows that troubled Cuiviénen. Then the Valar rejoiced, and yet they were in doubt amid their joy; and they debated long what counsel it were best to take for the guarding of the Quendi from the shadow of Melkor. But Oromë returned at once to Middle-earth and abode with the Elves.

Manwë sat long in thought upon Taniquetil, and he sought the counsel of Ilúvatar. And coming then down to Valmar he summoned the Valar to the Ring of Doom, and thither came even Ulmo from the Outer Sea.

Then Manwë said to the Valar: 'This is the counsel of Ilúvatar in my heart: that we should take up again the mastery of Arda, at whatsoever cost, and deliver the Quendi from the shadow of Melkor.' Then Tulkas was glad; but Aulë was grieved, foreboding the hurts of the world that must come of that strife. But the Valar made ready and came forth from Aman in strength of war, resolving to assault the fortresses of Melkor and make an end. Never did Melkor forget that this war was made for the sake of the Elves, and that they were the cause of his downfall. Yet they had no part in those deeds, and they know little of the riding of the might of the West against the North in the beginning of their days.

Melkor met the onset of the Valar in the North-west of Middle-earth, and all that region was much broken. But the first victory of the hosts of the West was swift, and the servants of Melkor fled before them to Utumno. Then the Valar passed over Middle-earth, and they set a guard over Cuiviénen; and thereafter the Quendi knew nothing of the great Battle of the Powers, save that the Earth shook and groaned beneath them, and the waters were moved, and in the north there were lights as of mighty fires. Long and grievous was the siege of Utumno, and many battles were fought before its gates of which naught but the rumour is known to the Elves. In that time the shape of Middle-earth was changed, and the Great Sea that sundered it from Aman grew wide and deep; and it broke in upon the coasts and made a deep gulf to the southward. Many lesser bays were made between the Great Gulf and Helcaraxë far in the north, where Middle-earth and Aman came nigh together. Of these the Bay of Balar was the chief; and into it the mighty river Sirion flowed down from the new-raised highlands northwards: Dorthonion, and the mountains about Hithlum. The lands of the far north were all made desolate in those days; for there Utumno was delved exceeding deep, and its pits were filled with fires and with great hosts of the servants of Melkor.

But at the last the gates of Utumno were broken and the halls unroofed, and Melkor took refuge in the uttermost pit. Then Tulkas stood forth as champion of the Valar and wrestled with him, and cast him upon his face; and he was bound with the chain Angainor that Aulë had wrought, and led captive; and the world had peace for a long age.

Nonetheless the Valar did not discover all the mighty vaults and caverns hidden with deceit far under the fortresses of Angband and Utumno. Many evil things still lingered there, and others were dispersed and fled into the dark and roamed in the waste places of the world, awaiting a more evil hour; and Sauron they did not find.