True Queens Harbinger (question)

By scrounger, in Game of Thrones: The Card Game

Kneel True-Queens Harbinger to lower the cost of the next Dothraki or Dragon character you play or put into play using Ambush this phase by 1.

Does this mean only ambush cards I have in my hand can be used or can I use any card at all in my hand for ambush??

Thanks in advance!