Corpseblight

Built upon harvested spores from a carefully curated fungus, this is a powdery contact poison often thrown in a glass vial, where it affects anything within a 5' radius of where it shatters.
Any undead creature that is exposed to Corpseblight may take necrotic damage that ignores resistance and immunity.
Every hour afterwards, the undead may continue to take necrotic damage. The undead may be permanently infected if not strong enough to ward of the blight. If the undead creature is destroyed, they disintegrate entirely. If Corpseblight is applied to an inanimate corpse, the corpse disintegrates over 24 hours, leaving no trace.
Corpseblight is infectious while active. Any applicable undead that comes within 5' of an afflicted undead or corpse for the first time may become infected aswell. Successfully throwing off the infection does not grant any sort of immunity to Corpseblight. Thus, a large mob of undead will often repeatedly reinfect one another
Corpseblight is a very rare fungus that one has to create more of by 'breeding it' on actual corpses

Scientific Name
Cadens cadaver