Fungal Poison is an ingredient in Agrippa's Potion. It takes the role of the paralyzer mentioned in the formula.
It can be obtained from a mushroom located in the Choir, outside the room with the Brazen Bull. The mushroom containing the poison gland can be broken open in many ways, including using the hammer and chipper on it, throw a rock nearby on the fungi, jumping on it, or just clicking on it repeatedly.
If it is not acquired before the shadow blocks the entrance to the Choir, it can be found next to the rocks that block the passage.