How about "can't attack or block unless"? That'd still be worse than Pacifism, but this is worse than Guard Duty.

uh. I forgot the "block" bit.

It's a pity that this is so much worse than Pacifism. I wish there was some way to decrease its cost, but there clearly isn't. Maybe it needs some small extra benefit?

It's still worse than Pacifism, though, because it 1) is a gold card, 2) gives the opponent a choice, and 3) is uncommon.

Yeah, even "unless its controller sacrifices all other permanents" would still be a Pacifism that cost rather than , and wouldn't actually stop Blightsteel Colossus from hitting you.