From the introduction:
The focus of this adventure is the horror-comedy of its titular villain. Velkis himself is a fairly complex creature, so several pages are devoted to how to run him. The Bloodsoaked Boudoir should not be run as a stand-alone adventure.
Instead, Velkis can be added to the encounter table of an area the PCs are already interested in. He requires many victims, so he may capture someone the PCs care to rescue. Failing that, if the players encounter him alive after they’ve slain him, their curiosity will be piqued. By the time the players find the entrance to the boudoir, they should already have a history with Velkis.
This adventure is based on Lamentations of the Flame Princess rules, but should be compatible with any old-school fantasy dungeon crawling game. For those using other systems, note that I assume unarmored AC 12, and a silver piece standard for treasure.