This is just my opinion, but I think the unicorn is a little over powered, with having unmatched speed, nullification abilities for long periods of time, and the ability to summon powerful natural disasters at will! To top it off, it's unkillable, which strike as a bit too much for a creature this string in the hands of anyone except the GM. I've no problem with a creature being strong, or stronger, than dragons, but this one is God-level with all of it's abilities combined. And one of the rules in the CS was to explicitly avoid that. 

I know that it has already been approved, but I really implore you nerd it's abilities a little, like making Nature's wrath involuntary, lessening the length of time on the nullifies(I personally would like this to be cut, but that's because I just dislike total nullification powers in general), and/or making it killable in a realistic way. Basically, I see too many strengths in this creature and no real weakness or restraints to balance them. Once again, this is only my opinion, take it and do with it what you wish.