Gal-en-du'un is a type of yellowish-green lichen which is founded on Vulcan's volcanic plains. When consumed, either through ingestion or by burning the plant and breathing the fumes, it causes strong hallucinations which last several minutes in duration.
"Dream moss", as it is known, was used in ancient times by ecstatic priests to induce revelatory visions. Interestingly, its effect on Vulcans is much stronger than its effect on Humans. Its modern uses are restricted to medicine, as a number of Vulcan analgesic and neurochemical treatments use gal-en-du'un essence. (Last Unicorn RPG module: The Way of Kolinahr: The Vulcans)