[ wryly ] I believe the line in the canto runs:

A night there was when winter died;
then all alone she sang and cried
and danced until the dawn of spring,
and chanted some wild magic thing
that stirred him, 'till at last it broke
the bonds that held him, and he woke
from dreaming deep and cold despair.


... It is a long tale, but it is said also that Luthien was as enchanted by he, and great was their doom indeed.