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!

It means that the next time you play a character or use the Ambush keyword on a character (with the appropriate trait, of course) in that phase, you pay one less.

This effect only lowers the cost. It does not allow you to play cards or use Ambush in any way other than the normal way.