Arthur was born of magic by Nimueh. But to give life, life must be taken, that is how Arthur's mother died. Uther blamed Nimueh and so killed all her servants and friends, this is when he started to hate magic.
The knight is a creature conjured from the dead by magic. He is called the Black Knight and he is Arthur's uncle (the brother of Arthur's mother). He fought Uther to gain revenge he thought he killed his sister, and then when he was dying he swore he would return.