- Not to be confused with Black Mushroom.
It can only be obtained by accessing the Black Mushroom curio in the Labyrinth Level 9, starting the hidden quest for Wirt's Leg (Diablo III) and Royal Calf. Bringing this to Adria's Hut and accessing her cauldron will produce the Witch's Brew.
A corrupt stench lingers around this mushroom, as though it is more than mere rot. Mixing it in a cauldron will yield a special brew.