The following information is based on events in Interview for a Wizard and contain spoilers.
This unnamed potion made from Dragon Breath is capable of killing one or more people instantaneously. This is presumably accomplished as a form of curse or spell unleashed from it rather than having to be consumed by every victim as it is considered a weapon of mass destruction by the Department of Magic, who has outlawed it. This application is also the reason that the Dragon Breath herb is illegal.
This potion is created by mixing Dragon Breath a certain way with several other items and various spells.