What female figure, according to the Green Knight, dwells in the castle? Why did she send the Green Knight to Camelot?

Morgan le Fey.... her intent was to hurt Guineve.... to cause her death.

She put this magic upon me to deprive you of your wits,
in hope Guinevere to hurt, that she in horror might die
aghast at that glamour that gruesomely spake
with its head in its hand before the high table.