Ghlen Livid is a spirit made from the freeze distilled Vul Nut, a reannual plant. Reannuals travel back in time, so Ghlen Livid can give its drinkers a vision of the future, which is, from the nut's point of view, the past. (This temporal anomaly is why Pteppic did his Final Run under the added handicap of a hangunder.) It has a distinctive green colour and is brewed in the Rehigreed province of the Agatean Empire. It is first described in The Colour of Magic.
A reference to Roundworld's The Glenlivet. According to empirical tests the Roundworld version doesn't show any useful visionary effects, perhaps even tending towards the opposite. But it is rumoured that extensive tests are going on nevertheless.