This myth was started on YouTube by an unknown person. The mansion is accessible and is used in several missions. It is located on the Prawn Island, right next to the Mendez Mansion. It consists of two floors, a big room with an overturned table and some broken chairs.
A hidden room is also located in the Mansion, which serves no purpose in the story line except for a hidden package location. Players have theorized that the ghost supposedly spawns in the room. Another strange fact is that the Ghost World can be seen slightly above the room. However, it's inaccessible.
Some players claim that the mansion is haunted during the stormy and foggy nights. Some profess that there will be audible voices and screams when inside, and some have said to see ghost orbs. Some players even included a modified Slenderman in the mansion. But none of these reports have turned out to be authentic including the ghost of Louise Cassidy.
As seen during the story line, this mansion is used as a gang turf. Gang hubs are often murky, usually to keep civilians away. During the mission Phnom Penh '86, multiple gang members are killed in this area, and the Haunted Mansion itself serves as the main building for the massacre. Another explanation is that big, empty, wrecked houses are generally creepy. This creepiness is increased when there is stormy and/or foggy weather, causing and misleading players to think that the mansion is paranormal.