Well, as the story goes, Uther fell in love with a lady called Ygraine, but she was married, so he started a war against his husband. One day, where everyone was distracted, you used your magic to have Uther take Ygraine's husband place and that's where Arthur was born from. Ygraine's husband died the next day and Uther married her. But they'd agreed to give the baby to you, and you gave it to a dude named Hector to raise him. When Arthur was old enough, you took the kid to pick up a sword in a stone and reclaim the throne.