[Aarthas]

Yes you're right it does require 1 unholy and 1 frost rune. The issue though is that the timer makes you think you are going to need to wait longer then you really do. It starts counting down from 5 seconds on some occasions, leading you to believe you have 5 seconds until your next scourge strike. Then after 2 seconds of countdown, when there is the number 4 on top of scourge strike on the action bar, it just refreshes and is ready to use again. The timer numbers on top of scourge strike just disappear, they don't change color and continue to countdown. It just disappears and scourge strike is completely active and ready to go.