Alchemy menu seen at the start of the game.
The Alchemy menu is where Plague Knight alters his loadout, similar to how the other protagonists change their currently equipped Relic, Curio, or Heirloom. In the Alchemy menu, Plague Knight can apply Health Tonics, switch which Arcana is equipped, and alter the Casing, Powder, and Fuse of his Bombs.