It adds a new "Van Helsing" themed playable character, a new level set in a haunted castle, and a trusty dog companion.
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. download-party-hard-dark-castle-the-games-download-exe
You can find the DLC directly on the Party Hard: Dark Castle Steam page. It adds a new "Van Helsing" themed playable