This substance is a meat dish made from a poisonous animal.
In the wild, this animal exudes some poison over its skin in order to protect itself from predators, in addition to using it on its prey. As with most venomous species, the antidote is also obtained from the same animal; they need not to intoxicate themselves with its poisoned prey, so they naturally a generate an antitoxin as well, which is present inside their body.
This is a very appreciated dish by the noble families of this country, with its own protocol on how to eat it (just as most of their plates). As it happens, the guests will be eating a poisonous, outer piece, then an inner one afterwards which neutralizes it, and so on. This mixture is what provides its particular taste, so highly valued (actually the consequence that you start getting poisoned then immediately an antivenom, then some more poison and so on). The wine that is traditionally served with this dish also happens to cancel some of the poison activity.
It is also possible that some of the noblemen which use eat this animal on special feasts, have developed some immunity to this poison.
However, if someone from the group of the newly-arrived ambassador which was being honored with this summum of their culinary art didn't eat the whole piece served, or was picky about the parts they took… they could have ingested a lethal dose of the poison, but not enough antivenom to counter it.
That could cause quite a diplomatic issue.
Note that the aristocracy doesn't generally know the details about this, or that this dish involves taking a poison. From time to time, someone gets a bit ill after a banquet, but given the amounts of food served, that's not surprising.
Also keep in mind that this animal needs to be cooked right. If you overcook it, you would cancelling the outer venom, and it would lose its special flavor!