The Itempedia of the Nightmare is a hidden category separate from the Frontier's Itempedia. It can only be unlocked after the players had accessed the hidden room in The Deli and pressed the orange button. The players should then return back to the place where they were originally teleported from and find a door near it (a door with an orange hexagon-like eye above it).
Unlike the Frontier's Itempedia, the Itempedia of the Nightmare houses many collectibles from different categories of the game, from consumables to monster drops, from plant to garments. Most of the skills learned from all four sections of the Frontier's Itempedia are required to fulfill the completion of it.
List of Known Collectibles
There are currently 72 collectibles in total that are categorized under the Itempedia of the Nightmare. This table will be sorted in the order it goes in.