The Hidden Library, introduced during the Ratboy's Nightmare Update, is a location that players can explore in The Nightmare. It can only be accessed once the players had pressed the orange button located in a small hidden room in The Deli.
After the players had done this, they may head to the door closest to the first room that the players were teleported to. Interacting with the door with an orange hexagon sign above it will trigger a teleportation spell, taking the players to The Hidden Library, where they may find Rat Tokens and the Itempedia of the Nightmare.
Two doors can be seen behind the Itempedia of the Nightmare's pedestal, a red door, and a purple door. The red door can be unlocked once the players are halfway done with their Itempedias of the Nightmare while the purple door can only be unlocked after the players completed the entire Itempedias of the Nightmare. The red door leads to a small room where they can interact with the vendor, Rising Waters, to purchase a variety of items from the NPC. The purple door leads to a small room where they may pick and acquire the Falling Light Set attached to its armor stand.