Just take a look around man, see what calls to you.
The Dispensary is run by Destiny the faerie, who, despite having no particular need to commune with mortals, is driven by her passion for certain herbs and "good vibes" to run potion shops throughout the dungeon.
Dispensary appears on the 5th floor of every biome (floor 5, floor 15, floor 25, etc.)
Destiny has up to 3 products available to purchase with gold at each location: HP Potions, Potion Slots, and Dispel Curses.
The price of HP Potions increases with distance reached.
Note that there is a maximum of 5 total potion slots. Destiny will not sell you slots after you reach that limit.
