Elixir of Passage
From Mistipedia
The Vistani sell, at the discretion of Count Strahd von Zarovich, an elixir that allows safe passage out of Strahd's Choking Fog that rings the Village of Barovia and Castle Ravenloft.
The elixir's recipe is a secret. A Vistana can manufacture the elixir with a successful Craft (Alchemy) skill check at DC 25. The elixir remains effective for 10 minutes.
