You can find the DLC directly on the Party Hard: Dark Castle Steam page.
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. download-party-hard-dark-castle-the-games-download-exe
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. You can find the DLC directly on the