Somehow, Magnus has never put 'knight' and 'genuine danger' together in his mind. Which is dumb. He's fought with Sagramore, he knows how good he is. But it's all been kind of abstract. You think of a knight, you think of, like, jousting for fun or whatever, not genuine fighting.

He doesn't like the thought of Sagramore almost dying!!! It's not a fun little thought! This is probably very evident from the set of his face as he says, "Oh."