Well, as far as I know, you must mean an "Elixir of Shadow Power," which is made in Alchemy. Same with Shadow Oil. Otherwise, it's probably a quest item, and you should just check your log, where it will tell you ^_^
Shadow Oil's recipe is sold by a vendor in 1k Needles (Horde) or Duskwood (Alliance)
Elixir of Shadow Power is sold in Stormwind or Undercity.
Hope this helps ^_^