Family Tree Trivia

[alaklondewen]

Symestreem is right. Here is the passage:

[Luthien] was not daunted by his eyes; and she named her own name, and offered her service to sing before him, after the manner of a minstrel....Then suddenly she eluded his sight, and out of the shadows began a song of such surpassing loveliness, and of such blinding power, that he listened perforce; and a blindness came upon him, as his eyes roamed to and fro, seeking her.

She did indeed sing for Morgoth. She sang so powerfully that she was able to lull him into a dream.