Overcosted? It's a NASTY deterrent; but if they can kill it outside of combat a useless one. (And it will usually die after damage is dealt, making it a little bit too late)

I think it's about right. (It could alternatively be cheaper but only destroy one creature.)

At worst it's "destroy an attacking creature with power four or greater, even if it has hexproof" which is a bit expensive at 3WW, but would probably see play. But this gives you a lot of other options: it will usually destroy at least one other creature, and you can always play it as a wall if your opponent has a 3/3. I agree 5-cost is a little clunky (4-cost and slightly less variable might be a bit better), but I think the cost is about right for what it does. If it destroyed all attacking creatures before combat it would

Actually yeah - the current thing lets you, for example, cast it as a deterrant, then unsummon it during some other attackers phase in multiplayer...

So it probably can't be overcosted. And I don't think quite this has been done before, and it's thematic. So 5 I guess it stays.

Oh yes, I'd forgotten unsummon. I'm not sure if you should be able to unsummon it, that needs two cards, but is really good if you get them because you can destroy all attacking creatures once (twice if you have unsummon with flashback), and then play the wall again.

I kinda want this to be that nasty though. I really dislike the idea of costing a wall at 6, and it requires some setting up to get eeevil use out of it. I think I'll leave it. (But if someone manages to give this old-fashioned-phasing-on-demand it's just scary)

Yeah, it's hopefully ok as a scary uncommon, I was costing it at as an average.

This, along with several other walls, needs to be an artifact. The 'flash' part does make surprisingly little sense.