Witch's Brew is a quest item in Diablo III. It is only available during The Darkening of Tristram in January.
It can only be obtained by finding the Rotten Mushroom and using it on Adria's cauldron. Bringing this to Farnham's corpse will trade it for Drunkard's Debt.
Item Description[]
Powerful Elixir
This strange concoction has been steeping for several years now...