Usually discovered on an old hard drive or sent by a "friend" who has since disappeared.

In the early 2000s, files like this were often "jump-scare" programs designed to maximize volume and flash disturbing images.

A of a specific real-world malware or game with this name?

Hackers often name files after popular creepypastas or urban legends to trick curious users into downloading trojans or ransomware.

In the ecosystem of online horror, a .rar file is rarely just a compressed folder; it is a Pandora’s box. "Nightmare Mansion" is frequently described as a top-down RPG or a point-and-click adventure found on obscure forums or defunct file-sharing sites. The Core Narrative Tropes