While some third-party sites list an executable named PartyHardGame.exe , it is highly recommended to use official launchers like Steam or GOG to avoid security risks associated with unofficial .exe files. Technical Tips
If the DLC doesn't appear after "purchasing" (it's free), try logging out and back into your Steam account or verifying the integrity of game files.
You typically need to own the original Party Hard game and, in some cases, beat the first level of the main game to unlock the Dark Castle stage.
It was developed by Pinokl Games and published by tinyBuild. How to Access the DLC
If you own the base game on Steam, the DLC is often added to your account automatically or can be toggled in your library's DLC list.
It adds a new "Van Helsing" themed playable character, a new level set in a haunted castle, and a trusty dog companion.
Rather than searching for a standalone executable ( .exe ), the safest and most standard way to access this content is through the base game's official store pages.