Mordred as a young boy keeps calling Merlin "Emrys", which is actually Merlin's name as a Celtic druid and sorcerer. In Welsh, Merlin's full real name is actually Myrddin Wynn Emrys. Myrddin is his first name, Wynn is his family name and Emrys is his druid name.
In the original Arthurian legend, Mordrid and Arthur are half-brothers. Both their fathers loved Ygraine (their mother). When Mordrid's father was out fighting a war, Uther and Ygraine conceived Arthur. To hide what he did, Uther sent Mordrid's father to the front line to be killed. Once he died, Uther and Ygraine married.