In this book we explore the wonderous machinations of Mechanus and the creations of The One and the Prime. With What Was Left Behind 2: Inevitables, you receive every single Hierarch modron from Decaton to Secundus, a vast array of Formians (A race of hive-minded ant people), a player race with mechanical ancestery known as the Mechanatrix, and the guardians of Cosmic Law themselves The Inevitables. Topping it all off is the ruler of Mechanus, Primus who features Mythical Actions and a swath of unique abilities. The book is meant to expand upon the vanilla modrons as well, introducing the concept of Promotion. When one modron dies a modron of lesser rank takes the place of the higher one. This trait is touched on in the lore but was never truly captured in their statblocks.
- from the publisher's blurb