Don't agree it is too fast for a poison - some real ones are ridiculously deadly.
It could also be non-magic if one believes in spontanous combustion being a real thing and this being a substance which triggers such an event. Exploding is a bit much, though.
It is, however,a bit too easy to create for the level of power it has. Whipping it up under duress seems unlikely as well.
Also, those with low BMI might not be killed :)
It does need a little more fat to it though, Moonhunters requests would help flesh it out.
This is a simple, but rather cruel poison. When the full dose is administered, the sensitivity of all nerves and senses is heightened and continues to increase in sensitivity over time. Eventually even the sound of one's heart will seem deafening, the taking of a breath excruciating, and the light of a single candle blinding.
In the end, the victim will be reduced to silent, quivering wreck who will beg for death, should they manage to speak at all. Perhaps fate will step in and still their suffering heart.
Fortunately, the crafting of this toxin is difficult, expensive, and requires exotic components. The antidote is also similarly difficult to craft. Go to Comment
This toxin, derived from the fairly common medicinal herb Jacobs Root, has a very subtle but possibly devastating effect. It severely impacts night-vision (not thermal or similar exotic vision types), essentially blinding most creatures after nightfall. Duration of the blindness is proportional to dosage. Even full moonlight is still terribly dark to the victim, and in anything less bright, it is pitch black.
The toxin has a faint, carrot-like flavor and is slightly bitter.
There are rumors that a preparation with permanent effects can be brewed, but requires much tedious effort and additional rare components. It is an effective means to destroy many criminal's livelihoods. Go to Comment
Your point about countermeasures is well made, and could help reduce it's impact. However a powerful nation could get around some of the measures by forging or stealing watermarked paper and then treating it.
The wax seals and Notary are an excellent countermeasure.
Regarding damage transferral, it was not stated explictly, but only minor alterations such as writing will transfer, otherwise the use as a 'backup' copy is not possible. So, I'd say physical damage (tearing, etc) do not transfer at all, and total destruction like burning will simply cause the other sheet to become somewhat scorched but readable. Go to Comment