Morgause is the older half-sister of Supreme King Arthur. She was married to King Lot of Orkney, and mother of Gawain (and his brothers). 

King Lot joined the second rebellion against Arthur and was killed in the battle by the knight Pellinore.  

Pellinore was a father to Lamorak, a beautiful, young man, who courted widowed Morgause, became her lover, and made her pregnant. By the pregnancy alarmed Gawain. He demanded to know who is the father, and when he learned that his mother slept with the son of a man who killed his father, he got furious. 

Gawain set a trap on Lamorak and killed him insidiously. He wanted to kill also Lamorak’s baby, but Morgause showed him a dead kid – the baby died during childbirth. 

Pellinore is still searching for who killed his son.