Answer:
Sir Mordred was Arthur's illegitimate son by his half-sister Morgause.
The wizard Merlin prophesied that Arthut's son would kill him, for which the King demanded all babies to be murdered.
Mordred survived and was raised by a fisherman and his wife. He was later taken by a sorceress named Nyneve to live with his mother in the Kingdom of Lothian. At age 15, he went to serve as a knight to King Arthur.
However, he betrayed Arthur and fought him in the Battle of Camlann, where he injured his father fataly before dying, fulfilling Merlin's prophecy.