Does “you might have known” here mean “probably you already know” or does it mean “this is a no surprising fact for you that”?
“You, Sir Caspian,” said Aslan, “might have known that you could be no true King of Narnia unless, like the kings of old, you were a son of Adam and came from the world of Adam’s sons.
I think in this context it means he probably already knew.
“I might have known” can mean it is no surprise. This is tricky to explain. When spoken aloud the tone of voice and body language are important. This is usually said when the results of someone’s actions are negative. It’s like saying that someone’s actions were stupid, and the person has a habit of doing stupid things. So it’s no surprise that AGAIN they did something stupid and it turned out badly. It’s predictable because of their past history.