You are seriously overvaluing this effect. On 1vs1, Sleep gets this done and more for a lot less. There's a lot of cards you can compare this to, but they all say it's overcosted and of too high rarity. Archetype of Imagination, Ruthless Invasion, Panic Attack, Ghostform, Open into Wonder, etc...

Also, "unblockable" isn't a thing in MTG anymore.

Still better than Part Water. ;)

Mmm, this is a "break the groundstall" card, which is a good thing to have in a set, but it should cost four or five, and be uncommon.

Unblockable is a bit of a harsh thing, though. Maybe flying?

Strangely, I can't find any cards that would just give all your creatures flying UEOT. There's Magnetic Flux, but that is just garbage.

­Levitation says that such an effect could certainly cost less than 4 mana.

I know I read somewhere that because Sleep and Overrun were both in Magic2010, Development forced Negate into common so there was at least one good answer. I can understand why a person would want Sleep variants at rare. "I play a bunch of creatures and then you lose" gets old when it happens too often.