Dragonbane

Dragonbane is currently the only known poison capable of killing dragons. It contains powerful chemicals that destroy the dragon's immune system. Strangely, it only affects dragons; humans who ingest it only feel a slight spicy, burning feeling in the esophagus.

Dragonbane was used to kill Prince Greatjaw when Prince Grimgnash poisoned his evening wine. There are several other recorded incidents, mostly performed by poachers.

The shadow dragons are currently working to find a cure for dragonbane but so far nothing has been found. However, the infinity dragons may have already found an antidote, as there are no recorded incidents in history of an infinity dragon being killed with dragonbane, although several admit to having tried to kill them with it. This could be because they are immune to the effects. Dragonbane doesn't affect acid dragons, since they are naturally immune to all toxins.

Ingredients
There are several ingredients used to make dragonbane poison: first, take the ground scales of the type of dragon you want to kill, second, three drops of mercury and finally add one gallon of any substance poisonous to humans. Then it must be left to boil for one month and kept hot until use.

After analyzing small samples of leftover dragonbane researchers have reported these three substances were used in all confirmed cases. There may be more ingredients in specialized types of dragonbane but so far there is no evidence to support this.