In early writings there was a concept of thrall Noldori in Hithlum and more broadly across the Great Lands, or what came to be known as Middle-earth.
These were not thralls in the traditional sense of captive elves in Angband, but instead it was being in a dream-like state and moving freely, while in fear and doing ill deeds to the benefit of Morgoth.
This concept was included in the Tale of Turambar where they felt the eyes of Melko burn them from afar. These elves were in the woodlands, and would ultimately assist Tuor at the behest of Ulmo.
In his tale it said that they did service to Ulmo while in fear of Melko.
Finally it said the Maeglin was under it as well in betraying Gondolin.
Melko’s means of achieving this was the Spell of Bottomless Dread. It was a spell cast broadly, diffused across the land, and less a tangible thing, more a state.
It appeared to spread and afflict as the Evil Breath, but on Elves instead of the very young or old. It worked like the curse on Hurin’s family as it shifted one’s actions and intentions to Melko’s intentions and ends despite one’s own best intentions.
In the earliest awakening of the elves, many never made the full journey and there were those who were lost. Some were known to be physically taken, but others likely fell under this spell.
Despite being referenced in early versions of each of the three Great Tales of the First Age, it was never specifically told when and why the spell was first cast.
It does seem that it was cast by Morgoth before the Battle of Unnumbered Tears, for the references of it never fully left the tales, though it was not called out by name in later versions as it was in the early ones.
The Spell gives validation to Melko’s power in the First Age and its function was part Evil Breath and part Curse. The chief difference between the Spell and Curse was that the spell was generally applied like the Breath, whereas the curse was focused and directed on specific individuals and those who interact with them.