Morgause comes from Arthurian mythology. She has been said to have been either
Arthur's half-sister or his aunt. In some stories, she is said to be the mother of her half-brother's son, Mordred, in other, she fostered him for her sister,
Morgana. The name is pronounced 'mor-GOHZ' and as for it's origins, my research leads me to believe that it's Breton.
As for
Margaux, I like it, but at the moment, not enough to consider using it.