Exava, Devourer of Souls

First Strike, Trample, Haste
Unleash (You may have this creature enter the battlefield with a +1/+1 counter on it. It can't block as long as it has a +1/+1 counter on it.)
Whenever an attacking black or red creature you control deals combat damage, double the amount of damage dealt unless the defending player pays {2} for each attacking creature.