Positoxins were first introduced in the 3.5 Book of Exalted Deeds - the opposite tome to the wonderful Book of Vile Darkness. They were basically poisons that were dangerous to the undead, that paladins could use without taking the shine off their halos. Here is a quick one I dreamed up whilst brushing my fangs...enjoy!
Star Rose Tincture (Positoxin – Rare Consumable): As a general rule the undead are immune to poisons. However, there are certain substances, distilled from celestial materials and infused with the radiant energies of life, that act just like toxins to them. These “Positoxins” are rare, but valuable substances that can greatly aid those that hunt the unquiet dead.
Star Roses are a species of empyreal flower that sprawl across the lower peaks of several heavenly mountains. Their soft blooms glow with a blue-white light, much like moonlight, and their pollen fills the fragrant air with tiny specs of silvery light. When the essence of these divine flowers is infused into blessed water, and mixed with certain other substances baneful to the undead, the result is this substance – a faintly luminous, clear positoxin, that smells quite strongly of rose blossoms, and is slightly thicker than water.
Harmless to the living, it deals 13 (3d8) points of radiant damage to the undead when delivered on a weapon (D.C. 15 Constitution saving throw for half). A vial of this substance inflicts 27 (5d10) radiant damage if smashed against an undead creature (D.C. 15 Constitution saving throw for half). This substance resonates at a strange dimensional frequency, and is able to harm incorporeal targets as readily as corporeal ones. In other words, such targets do not get their usual resistance to this substances damage.