From the introduction: Rappan Athuk is a sprawling adventure that occurs above and below ground, and possibly over multiple worlds. In addition, Rappan Athuk “sprawls” across several hundred pages and several editions of the world’s greatest roleplaying game. This chapter talks a little about both of these. The beginning of the chapter talks about the book itself – room numbering and descriptions, locations of monsters and magic items, and so forth. The middle gives some general information about the world, and the rest is about combining the fifth edition rules with an old school adventure.