The Fish Hall, introduced during the Ratboy's Nightmare Update, is a location that players can explore in The Nightmare. The Fish Hall can be accessed through a narrow hallway in The Mansion, on the second floor to the right.
Once the players enter the Fish Hall, they will see a huge storage room and hall of several fishes recognizable from the Frontier, such as the Stanfish, Proudfish, and Bass. Like The Mansion, the Fish Hall is also overrun by bizarre and terrifying creatures. Players can also find other collectibles of the nightmare in the Fish Hall, such as the Dream Dust and the Crumbling Root.
While in the Fish Hall, the player will take 100 damage from an unknown force every now and then, and armor will help reduce the damage taken. It's likely that this "unknown force" could be the putrid smell of the raw fish everywhere, literally intoxicating the player as they slowly take damage.
There are two hallways that split the main intersection of the hall, the left, and the right. The left hall will lead the players towards the other part of The Mansion, while the right hall will lead the players towards one of the entrances of The Grand Hall.