Keep the Magic Secret (слэш)

For the prompt: Someone tells Arthur about the legends of Emrys, an all-powerful warlock whose destiny is to protect Arthur and his kingdom and help bring about an age of peace. He is told that Emrys is someone close to him, and has hidden his identity and trials over the years to protect himself and make sure he can continue on at Arthur's side. When Arthur asks who it is, the person turns to Arthur and shrewdly asks: "Arthur, who do you want it to be?" ... Arthur's mind automatically goes to Merlin.