At one point it the Duke has a valid reason rather be due to the spoilers or not. Its known that the Duke only took Jonas in was because of the duchess. The Duke only married Vanessa due to a contract. (Anything for his love of his life)
However, due to the duchess love and care (obsession) she couldn't let go Vanessa and kept her son so she can have a part of her still with her. Which caused Jonas to be the next heir for the Duke. Either way Jonas was going to know about the cup. And it was mention that the oldest king doesn't even know about the power it has in the cup.
At one point it the Duke has a valid reason rather be due to the spoilers or not.
Its known that the Duke only took Jonas in was because of the duchess.
The Duke only married Vanessa due to a contract. (Anything for his love of his life)
However, due to the duchess love and care (obsession) she couldn't let go Vanessa and kept her son so she can have a part of her still with her.
Which caused Jonas to be the next heir for the Duke. Either way Jonas was going to know about the cup. And it was mention that the oldest king doesn't even know about the power it has in the cup.
Its interesting to connect the dots.