1) Cards which kill a creature regardless of their stats are a big no-no and are highly unlikely to be implemented in game.
2) Cards which can affect immortal and burrowed creatures are also a no go zone. Many people hate on Sow for this so we really don't need another point for dispute.
3) Water already has 2 AoE cards (Dry spell and flooding), I don't think it needs more. Not to say that this means there shouldn't be more, that's just my opinion.
4) This being a rare would make lower level players less likely to get one whilst higher level players can. This could upset the PvP 1 & 2 and possible the arena.
5) Even if this card costs 15
and is rare. It is waaaaaaay too powerful. <10% of deck strategies are creature-less so this would be game breaking.
6) Try to avoid double posting, you can always click modify post if you want to add something to your post. Especially if it's only because you forgot to say something (which is what it looks like)
I'm not hating on your idea, I'm just trying to help out. Cards which are very powerful can be hard to balance so they aren't abuseable.
