A set of supplements outlining obscure cults of the Old World. The supplements include setting documents written by a witch hunter named Samael Neuhauser describing a chaos cult. The supplements provide a fleshed-out cult with NPC cultists, artifacts, encounter outlines, scenario outlines, and advice on using the cult as part of a campaign.
Only two of the supplements are numbered: Volume 1 and Volume 3. Volume 2 is actually simply called The Snail Man. Accorsing to the author, in the Old World lore the document exists but was never finished (the supplement itself is a complete game supplement).
According to a blog post by Sami Uusitalo:
"The thing about the numbering and volume 2. After I had written this Lauri Maijala approached me and asked to write the next volume in the series. I had another idea in mind, so I asked him to use another Templar and number it 3. Then, when I got around to write volume 2, the core concept changed quite a bit and the document ended up being the fan supplement called the Snail Man (also written by Neuhauser). The idea in that document is that he didn't finish it, so it was just a long letter to a Witch-Hunter General, and thus not a finished volume 2 of The Blasphemous Cults."