Awww, you beat me to the idea. Though I was thinking of more of a sacrifice-reward theme of a demonic lord or something. Let me see what I wrote....
"When it comes into play, your creatures gain -1/-1 and opponent's creatures gain -2/-2." As a demonic lord or something, it likes sacrifices. So the -1/-1 is like giving your arm as a partial sacrifice. In return, the demonic lord or something curses the opponent's creatures.
Anyway, that was my version of this idea.
I don't see a problem with Prophet giving -defense, as long as the cost is factored in. Thunderstorm costs 2 (1), RoF costs 7 (5). I'd go with -1. So, counting the cost of blessing, 3, + 2 for cost of thunderstorm effect, and + another 1 or 2 for the 1/5 stats, and you have a cost of 5 or 6. If that's not enough to make rewinding + replaying multiple times in a turn, then add more or weaken the effect.