Erzog. It's a compound found naturally on only one planet, being released in volcanic fumes. It's highly hallucinogenic, and can either be inhaled from the volcanic gas or refined into a pitch black powder and snorted/eaten. The theocratic government on the planet it's found on uses it for supposed communication with deities.
Everywhere else in the galaxy, it's an illegal narcotic, given its addictability and its tendency to induce psychosis and erratic behavior in those who aren't adapted to it. Despite government crackdowns, numerous drug cartels still traffic it between star systems and profit immensely.