Tescari Mushrooms are a powerful narcotic produced deep within the caves of the Tescari Dominion. Knowledge of their cultivation is a closely guarded secret. Within Tescari assassin rings, the drug is forced upon young initiates in order to make them dependent upon their trainers, who control the only supply of the drug. The drug has been known to be fatal to those going through withdrawl, making it nearly imposible for users to break their dependency. 

The chemical compounds contained within the drug require a certain amount of alteration by Tescari casters in order to produce the desired effects.  This makes the drug extremely difficult to reproduce.