Concept: “From the darkness, they emerged. Of the darkness, they are made.” 
Content: The aforementioned fiends and related diseases, parasites, and tables.
Writing: Themes of infection, subversion, and infestation tie this book of beasts.
Art/design: A mmutilated mashup of tortured illustrations in a stylized but effective layout.
Usability: Rules and description separated and identifiable for ease of reference.