Interesting card concept! Might need a bit more clarity in order to actually work. Probably should be a sorcery, too, to make things easier. And I'm not sure to what extent you intend to randomize the blockers. Is this intended to randomize which creatures are selected to block while still allowing the choice of what they'll block? Or is it meant to randomize which defending creatures block which attacking creatures as well?

Yeah, I'm working on a better wording. The idea is to randomize which creatures block which. You would do something like flip the creatures available to defend upside down, shuffle them, the assign them randomly to attackers. If this would result in an illegal block, you would swap them randomly until either all attackers are blocked, or no defenders are left. I just need to figure out how to words this succinctly and clearly.

How about:

Cast ~ only if a player controls an attacking creature.

Each creature defending player controls blocks an attacking creature at random.

How is this?

Edited slightly.