I like the idea and flavour. Would be simpler templated as just "other creatures with shardmind get +0/+1" (where 1 is this creature's toughness), is that different rules-wise?

It's quite different. The interaction with Deathtouch in particular is very different with that change.

You mean, deathtouch would kill all of them? I'm interested in rules modifications that DO work differently like that, but that wasn't obvious to me: I thought of deathtouch (or damage marking) as acting on a creature, is the intention here that damage would be marked on all creatures as one?

Yes. All of your creatures with Shardmind have no individual toughness. They share a toughness that incorporates all of their base toughness and the bonuses to that toughness into one.

If I control two Diamond Vanguards, they are effectively 2/2s. Dealing two damage to either of them or one damage to each will destroy them both.

If I control a Diamond Vanguard and a Sapphire Bastion, then I have a 2/4 and a 0/4. Four damage dealt to either of them, or any combination adding up to four or more damage that is dealt to both of them, will destroy them. A Pyroclasm would result in both creatures dying, for example. However, if Diamond Vanguard is dealt 1 damage, it will not die.

If I control a (Diamond Vanguard and a Sapphire Bastion, I have a 2/4 and a 0/4. Then a -1/-1 counter is placed on Diamond Vanguard. I then have a 1/3 and a 0/3. They both survive, though Diamond Vanguard's contribution to the cumulative toughness is negated.

If I control three Sapphire Bastions and a player casts Languish, they all get -4/-4 UEOT. Even though their combined toughness is 9, they will all die. The Shardmind is getting a total of -12 to its toughness. This is significantly different from the effect Languish would have if shardmind instead gave +0/+X.

If I control an Ebony Demon, I'll need at least 7 points of toughness among my other shardmind creatures (given no other bonuses) in order for them all to survive.

OK, I think I see. That is interesting.

­This is significantly different from the effect Languish would have if shardmind instead gave +0/+X.

I'm not sure I get the same answer for all the specific examples though. I thought in this case, Languish would make all the Sapphire Bastions -0/-1, and then they would get -0-/-1 from each other Bastion, for a total of -3, which looks the same as your example. Does this happen in a different order?

I may have misinterpreted your intent behind a revised effect. I assumed your version of the effect would be "others creatures you control with shardmind get +0/+X," where X was the creature's base toughness. This would have to be static. To have a value that changed with current toughness wouldn't work well with layers.

Ah! Thanks for explaining. I assumed the comp rules would say "equal to this creatures toughness", but the reminder text would just list what it was, since that's what would matter most of the time. But I kind of glossed over the difference, that didn't make it very clear, oops. (And I'd forgotten mass toughness altering effects would change that, so that probably wouldn't be enough.)

I'd thought it would work with layers, I assumed that "other shardmind creatures get +0/+X where X is this creature's toughness" would work with other mass -X/-X cards as they'd both be toughness altering effects. But I don't actually remember all of the layer model, is there a reason that doesn't work?

That said, with the way you intended it to work, "all get +0/+X" isn't that helpful, 'cos it doesn't explain how damage is also shared. But I don't find the current reminder text sufficiently clear either. Maybe something like "isn't destroyed until the combined damage dealt to all shardmind creatures equals their combined toughness, when all are destroyed"? I'm not sure if that's clearer or not (and it's different in the case where one of them leaves the battlefield for another reason), but I'm worried trying to share the damage will not find a good reminder text.

"Other creatures with shardmind get +0/+X, where X is this creature's toughness" doesn't work as a static ability. The easiest reading to understand is that it doesn't work because toughness is so mutable. Say you run down the sublayers that determine power and toughness, and you decide that your creature's toughness is 3. Okay, so other creatures you control with shardmind get +0/+3. But wait. Now your 1/1 with shardmind is a 1/4. So the creature you just finished checking should be getting +0/+4. Right?

Power and toughness are determined in the final layer. Static abilities that grant power and toughness boosts can reference only statistics determined in higher layers, or statistics that exist outside the layer system.

Shardmind could work in the way it is currently intended if it was worded as "You may redirect damage taken to this creature across other creatures with Shardmind you control." That way, you get the "You choose how damage is dealt" from Banding without the confusing attack phase of banding. (Which I am attributing as the idea for shardmind)

headache Hold on I think that ability can exist in the rules, because it all applies in "Layer 7c: Effects that modify power and/or toughness (but don’t set power and/or toughness to a specific number or value) are applied." and each effect is only applied once, in timestamp/dependency order. So it doesn't form an infinite loop. Is that right?? But it never WOULD be printed, because I was wrong, the effects will stack once, with two 2/1s becoming 2/1 and 2/2 from the first ability, and then 2/3 and 2/2 from the second ability, which is completely confusing to everyone. Is THAT right??

I thought the dependency system meant you could reference something determined in the same sublayer, even if in this case you shouldn't. But is there something else that prevents that?

Either way, I agree that wouldn't be a good rules interpretation after all. Thanks for being patient while talking it through.

Volvary: I was thinking that. I think it might be a good implementation. Although it's not the same mechanically, since if you have all X/1 creatures, any redirection would kill one of them, but I think Link's intention was nothing would die until the damage equalled the combined toughness.

That is my intention. Clearly, with this much confusion, the mechanic is not a good idea.

Well, most of the confusion was about my suggested alternative rules :) I'm not sure your original rules are clear either (that's why I suggested an alternative), but there might be an alternative that works.