Good timing: the poisonous item pack was updated right now to include mechanically new poisonous gas flasks! Only on itch for now, as nexus is having ban issues again:
The new mod framework 1.4 permits substantially easier editing of TEMP/TBLU files with its new entity.json file format if you want to check the internals out. I also released a modder tool to go along with it, download that too if you’re interested in mod creation.
To answer your actual question: it is the AmmoImpactEffect specified in the referenced AmmoConfig. That template applies the correct death effect, besides other things like emitting particles and attaching the visible dart to the character. The framework should provide all tools needed to replace that in a collision-free way, if you want to edit it for a different effect. 
Maybe a dance-inducing dart gun would be a fun joke item, if somebody wants to make a joke item pack?