The Pharmacopoeia is accessible from Floors 6-9. It consists of 2 levels.
While here, you'll be under the mind altering spell of the Spiridus. You will effectively be deluded (each monster will appear to be a different kind of monster).
There are plenty of potions to be found in the Pharmacopoeia.
The Spiridus can kill you before you even know you're in a fight with a boss. She'll appear disguised like any other enemy, but her incredibly fast movement will be a giveaway. The Spiridus has 20 speed and 20 dexterity making her more accurate and faster than almost anything in the game. To top it off, she does PSYCHIC damage. This will be especially deadly with low intelligence (i.e. vampire form). And the absolute worst thing that can happen is if the damage leads to you becoming "vulnerable", after which you'll take even more damage (as much as 50% higher).
High dexterity or a Deadeye potion and high intelligence will make a huge difference. A Clarity potion will let you see the Spiridus's true form and protect you from vulnerability. Make absolutely certain that you're ready to take on the Spiridus because there are not many easy escapes from the fight.