It's called COMES CHAOS. It is a campaign setting supplement for B/X. It is 64 pages long and features artwork by Kelvin Green. For the background/story of this work (started back in 2016...when I was still in Paraguay!) you can check out this old post on the subject.
Here's the marketing copy:
COMES CHAOS is a campaign setting designed for use with the B/X fantasy adventure game (and compatible retro-clones). It contains the information needed to transform your home campaign into a chaotic hellscape populated with mutants and demons, dark sorcery and depraved cultists.The book contains new ways to use old character classes. It contains dozens of spells of dark sorcery. More than 100 mutations and "gifts" of the dark powers. New combat options. Scores of monstrous foes and demons. More than 50 unholy magic items. Rules for creating chaotic wastelands, rules for corrupting player characters, rules for demons and cultists and for running a setting of ever encroaching chaos and the heroes that struggle against its rising tide. Information on running a villainous campaign for aspiring champions of evil, as well as guidelines for redeeming those who fall to darkness.A 64 page book, illustrated by Kelvin Green. Whether you're looking for a unifying theme for your fantasy adventure game, or simply want to spice up your campaign with demonic possession and vile enchantments, COMES CHAOS has plenty of demented ideas for your enjoyment.
Cheers, folks. Happy Thursday!