Certain internal organs of the deep sea blowfish contain a terrible toxin, distilled by the Assassins of Ankh-Morpork into the poison bloat. A very little bit of the natural toxin or the distilled poison, either ingested or put into circulation by a small prick, will make the victim instantly explode because all of the molecules in his body suddenly want to be many times bigger. Some high-class restaurants have a very expensive blowfish squishi in which a very skilled chef prepares the meal, making sure that it contains absolutely no traces of the toxic internal organs whatsoever. That way the dining guests do not become plastered all over the walls and other guests. To be on the safe side it is even better not to use any part of the blowfish whatsoever.
Based on the real-life fugu, which is also deadly, though less explosive.