Morgoth captures Elves as spies and slaves
Event Type: Military/Strategic
Age: 1st Age
Years: 0060 ~ 0545
During the Siege of Angband, I 60 - 455
"...in his rear and to the north Morgoth had no foes, and by that way his spies at times went out, and came by devious routes into Beleriand. And desiring above all to sow fear and disunion among the Eldar, he commanded the Orcs to take alive any of them that they could and bring them bound to Angband; and some he so daunted by the terror of his eyes that they needed no chains more, but walked ever in fear of him, doing his will wherever they might be. Thus Morgoth learned much of all that had befallen since the rebellion of Fëanor, and he rejoiced, seeing therein the seed of many dissensions among his foes."
The Silmarillion, Quenta Silmarillion, Ch 13, Of the Return of the Noldor
The aftermath of the Dagor Bragollach (Battle of Sudden Flame) I 455 meant capture of large numbers of elves:
"Many of the Noldor and the Sindar they took captive and led to Angband, and made them thralls, forcing them to use their skill and their knowledge in the service of Morgoth. And Morgoth sent out his spies, and they were clad in false forms and deceit was in their speech; they made lying promises of reward, and with cunning words sought to arouse fear and jealousy among the peoples... But ever the Noldor feared most the treachery of those of their own kin, who had been thralls in Angband; for Morgoth used some of these for his evil purposes, and feigning to give them liberty sent them abroad, but their wills were chained to his, and they strayed only to come back to him again. Therefore if any of his captives escaped in truth, and returned to their own people, they had little welcome, and wandered alone outlawed and desperate."
The Silmarillion, Quenta Silmarillion, Ch 18, Of the Ruin of Beleriand and the Fall of Fingolfin
Nirnaeth Arnoediad (The Battle of Unnumbered Tears) I 472
"The remnant of the Eldar of Hithlum were taken to the mines of the north and laboured there as thralls, save some that eluded him and escaped into the wilds and the mountains."
Morgoth sent great strength over Hithlum and Nevrast, and they came down the rivers Brithon and Nenning and ravaged all the Falas ... and the most part of Círdan's people were slain or enslaved."
The Silmarillion, Quenta Silmarillion, Ch 20, Of The Fifth Battle: Nirnaeth Arnoediad
It was during Nirnaeth Arnoediad that Gwindor was enslaved:
"'A wandering Elf, a thrall escaped, whom Beleg met and comforted,' said Gwindor. 'Yet once I was Gwindor son of Guilin, a lord of Nargothrond, until I went to the Nirnaeth Arnoediad, and was enslaved in Angband.' "
The Silmarillion, Quenta Silmarillion, Ch 21, Of Túrin Turambar
All captives were freed with Morgoth's downfall in the War of Wrath;
"Thus an end was made of the power of Angband in the North, and' the evil realm was brought to naught; and out of the deep prisons a multitude of slaves came forth beyond all hope into the light of day, and they looked upon a world that was changed."
The Silmarillion, Quenta Silmarillion, Ch 24, Of the Voyage of Eärendil and the War of Wrath