In this story, there is no magic death penalty for breaking an Unbreakable Vow, because the vows are literally unbreakable. Harry would cease having to be Harry Potter in order to break the vow, and maybe not even that would work.
I have looked through ever mention of unbreakable vows in the fic and not found anywhere where this is made explicit enough for much confidence.
Of course it’s possible that in-universe everyone knows that Unbreakable Vows survive death—but it’s far from obvious because surviving death is really rare. (And presumably no one has yet survived death using a Horcrux 2.0.)
″...so shall it be,” Harry repeated, and he knew in that moment that the content of the Vow was no longer something he could decide whether or not to do, it was simply the way in which his body and mind would move. It was not a vow he could break even by sacrificing his life in the process. Like water flowing downhill or a calculator summing numbers, it was just a thing-Harry-Potter-would-do.
I don’t think “It was not a vow he could break even by sacrificing his life in the process” means what I think you may think it does.
(I think it means something like “Harry can’t, and won’t, say ‘Oh, screw it, I’ll destroy the world’ at the price of dying. He simply, will not make any choice that in his judgement risks destroying the world”. Note that this leaves entirely open the question of whether anything could release him from this constraint. Of course the word “Unbreakable” in the name is something of a giveaway; but I am not aware of anything in either canon or HPMOR that rules out the possibility that such a vow is somehow tied to the vower’s brain, or ceases to exist on their death for some other reason.)
In this story, there is no magic death penalty for breaking an Unbreakable Vow, because the vows are literally unbreakable. Harry would cease having to be Harry Potter in order to break the vow, and maybe not even that would work.
I have looked through ever mention of unbreakable vows in the fic and not found anywhere where this is made explicit enough for much confidence.
Of course it’s possible that in-universe everyone knows that Unbreakable Vows survive death—but it’s far from obvious because surviving death is really rare. (And presumably no one has yet survived death using a Horcrux 2.0.)
I don’t think “It was not a vow he could break even by sacrificing his life in the process” means what I think you may think it does.
(I think it means something like “Harry can’t, and won’t, say ‘Oh, screw it, I’ll destroy the world’ at the price of dying. He simply, will not make any choice that in his judgement risks destroying the world”. Note that this leaves entirely open the question of whether anything could release him from this constraint. Of course the word “Unbreakable” in the name is something of a giveaway; but I am not aware of anything in either canon or HPMOR that rules out the possibility that such a vow is somehow tied to the vower’s brain, or ceases to exist on their death for some other reason.)