A rare hallucinogenic mushroom, favored by the hierophant of the Druids' Grove
The psilocybe is a mushroom which druids employ in their rituals, it is not at all useful in the alchemy witchers practice. The Βοτανολογία skill is not required to gather psilocybes.
The hierophant asks Γκέραλτ to get a psilocybe mushroom so he can lure the remaining Salamandra into an ambush. Apparently this would make fisstech even more powerful than it is, so it makes for a very good bait for the planned surprise attack. This must be the distilled ingredient.