Here is the setup, I have the initiative as a quickcast action I choose to cast Magebane on an the opposing mage, but I do not reveal it even though I have the mana for it. During the quickcast the enemy mage wants to cast a spell.

The question is, can I reveal Magebane after he has said he will take a quickcast action also, in order for that mage to be damaged ?

P.S. I am sorry if this question has been asked already, if it has please direct me to the answer.