The Old Mansion is a medium-sized area split into two sections; the 'Side Halls' and the 'Forgotten Rooms'. It is infested with various creatures of the Nightmare, such as the Nightmare Lantern, Corrupted Onyx Knight, and Mansion Ant. Many collectibles can be found in the zone, such as the Plugstack, Clock, and Theatre Ticket.
There are two main sections of the Nightmare:
- The Side Halls can be accessed without the Mansion Key by traveling down a narrow stairway in The Mansion if the players took the door to the right of The Mansion's entrance (from The Deli) on the second floor.
- The Forgotten Rooms can be accessed after entering the Purple or Pink Door, however, players must have prematurely pressed the pink and purple button beforehand. After entering the doors, the players will be taken to the forgotten rooms of The Old Mansion. Players can find various mobs lurking in every corner, especially the large, empty room beside the smaller rooms. Entering the large, empty room will lead the players to a place where they can fight against various mobs that frequently spawn in the room. They can also find the red button in the room, found on the walls where the exit is located at.
Mobs that can be found in the area include Boomer, Corrupted Onyx Knight, Crimson Ogre, Gold Ogre, Golem Baron, Goon, Industry Giant, Macabre Candelabra, Mansion Ant, Nightmare Lantern, Path Gambler, Red Wolf, Spiker, and The Chewed.
The only NPC that can be found in the area in The Last Guard.
Overall, The Old Mansion is a sub-zone of The Mansion, acting as a precaution for players to prepare for dangers that can be found further in the Nightmare. The large, empty room acts as an efficient and excellent farming spot, due to the abundance of various mobs that spawns in the room. Some players sometimes check The Old Mansion in hopes of encountering a Theatre Ticket, especially if they are in the smaller rooms of the 'Forgotten Rooms' section.
- Half of The Old Mansion's area cannot be accessed without pressed buttons and/or required keys.
- The Last Guard is an NPC that can be found in The Old Mansion, presumably there to explore the area before his entire party was wiped out by the mobs.