I used this spell to try to win arena with some lower level heroes. Let's just say it gets you to the last fight, but it's not that effective against Umberdroths and Obsidian Golems. I think it they should probably make it not stackable, but it is the only Air Spell worth casting most battles. 

On another note, casting it 20 times in the same fight makes the game lag like crazy. 

What is it?

Obscuring fog benefits from air and water shards, but requires no rank in any magic school, so it can be cast by the sov or any imbued champion.

I guess it should require at least disciple rank in both water and air, as a way to balance it, since it is one of the best spells right now…