You know, I was looking at the mechanics page and thinking, "I don't remember making that card." No wonder! Negative toughness would indeed be interesting on a mythic demon.

From the way shardmind works, I think you can afford to reduce the negative health a lot since currently, if you have 7 total toughness and this, you have a bunch of creatures that share a single hit point (which is a feel bad moment I think). If this had say -2 on the other hand, you make it so it penalises your shardminds, forces you to already have 3 shardmind health for it to have 1 hp and then dodge spot damage.

I agree. I think the negative toughness is a bit too much. It's still an interesting concept, though.