The Potionarium is a location in the expansion Plague of Shadows. It is a hidden facility beneath the Village which Team Plague use as their base of operations to study Alchemy secretly and to develop the Ultimate Potion. To access the Potionarium Plague Knight must take the hidden route to Mona's room.
Once Plague Knight has visited the Potionarium, entering the Village from the map screen will bring him directly to the Potionarium; meaning he cannot access the Village. Each time Plague Knight defeats a member of the Order of No Quarter, he is automatically taken to the Potionarium where he hands the essences collected to brew the Ultimate Potion.
Upon entering the Village for the first time (forced after completing the Plains level), the guard Farrels stops the player, saying Plague Knight and his Alchemy aren't welcome in the Village. Should the player try to enter the Village, Farrels will stop Plague Knight and kick him out.
To enter the Potionarium, the player must go left, where Hedge Pupil will be, saying that the secret entrance to the Potionarium is blocked by barrels at his own house. By blowing up the barrels (and Hedge Pupil's house), the player can enter the secret passage below the Village, through the sewers. The path leads to Mona's room, where it is revealed she works with Plague Knight. By pulling a lever next to her desk, the two are taken to the underground lair that is the Potionarium.
Much like the Village in the original Shovel Knight campaign, the Potionarium is one of the two "safe zones" (other than the Armor Outpost) where the player can purchase upgrades from NPCs.
- Mona, who sells upgrades of bombs. By bringing her Cipher Coins, she can research for more of those items and increase her selling inventory. She also helps Plague Knight store the essences taken from the Order to brew the Ultimate Potion.
- Magicist, who sells maximum Magic Upgrades and Health Tonics.
- Mail Minion, who sells the Bait Bomb, used to fish Health Tonics in the pits seem in stages. There is a scrap sheet in the wall behind him.
- Percy, whose function is the same as the Bard. After Mona leaves the Potionarium, he will also sell and research items.
- He will be running over shall player move around rooms.
- Oolong, who plays the music once the Music Sheets are given to Percy (who in turn throws them away right at Oolong). By throwing a bomb on his funnel-like appendage, the player can obtain a Music Sheet. Two Cipher Coins can also be found by blowing up the wall behind him.
- Baz, who can become Plague Knight's minion after being defeated in battle, should the player talk to him again and choose "Recruit". He will give the player free Health Tonics when talked to.
- In addition, various minions can be seem wandering the Potinarium's left screen. When talked to, they will often give the player hints about the game. Players can blow them up off screen by throwing bombs at them.