Hello!

Sorry, if this already has been asked, couldn't find info.
Text: Response: After defenders have been declared in a challenge, kneel 2 influence or pay 2 gold to change the effect of winning the challenge to that of another challenge type.

Can I use it, if no defenders declared?

I believe so, because "after defenders are declared" is referring to framework action where your opponent chooses to declare defenders, they could of course choose to declare no defenders, then in the player actions between declaring defenders and before determining the challenge winner, you would play that response.

Although I guess it's a response, so you would play it in response to your opponent declaring defenders (which includes declaring NO defenders) and would therefore occur BEFORE the player action window I mentioned above.

declare defenders , you can declare or not, but this card you can use it even oponent declare 0 defenders, i believe its referred to windows defenders.

Point is, you are responding to the framework opportunity to declare defenders - which happens whether any defenders are actually declared or not.

Thank you very much for answers.