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In the earlier conceptions of what would become The Silmarillion, all Elven realms of Beleriand were destroyed much earlier than in the final work, and virtually all the Noldoli were enslaved by Melko as his thralls, working in the mines of Angband doing his work. The only free Noldoli were escaped thralls, who were mistrusted by the Sindar.
Melko's power in these early works is perceived as great: he is able to dominate the minds of the Elves wherever they go, by instilling his fear in them. Even the escaped thralls are thus not truly free. It is not inconceivable that in this early conception Thrall-Noldoli were used to breed the Orcs.
In the later material the published Silmarillion was based on, there are relatively few references to Morgoth's thralls. There are no clear mentions what happens to the Elves Morgoth captures (exactly where Finduilas and the other women of Nargothrond were taken to is never said), but the idea is still present in the end of the Quenta Silmarillion, which mentions that the Valar freed Morgoth's slaves.