To unlock this decoration or achievement at Call of Cthulhu you have to find all 3 books of unholy knowledge.
In the guide below you will find all of the books’ locations. This action isn’t required to finish the match, but it permits you to increase Occultism of the hero’s level, so it might be well worth doing it.
Read all of the novels of unholy knowledge
An overview of existing trophies and accomplishments can be found here in our trophy guide to Call Of Cthulhu.
Books Location Guide
Book #1: Chapter 6 – Office before the Gallery
Book #2: At the back of the bookstore’s main room
Book #3: In Doctor Fuller’s Office