From publisher blurb:
Extra-planar beings come in many forms, and for outsiders hailing from the lower planes, almost every one of them comes in some shade of evil.
From the vicious malevolence of the demons of the Abyss, to the merciless schemes of the devils of Hell, to the psychopathic sadism of the yamadutas of the Naraka, the planes provide an unending parade of cruelty, immorality, corruption, and depravity.
Collected herein are 50 fiends from the darkest reaches of the outer planes, including the Abyss, Hell, Naraka, Pandemonium, Limbo, the Dreamlands, and the Beyond (a terrifyingly remote place of madness, aberration, entropy, and pure chaos.
This book is designed for use with classic (BX/BECMI/LL) editions of the world’s original role-playing game, and with Old School Adventures Accessory PX1: Basic Psionics Handbook. It additionally provides information on dual-axis alignment, the planes of existence, languages spoken throughout the planes, and descriptions for a number of planes- and fiend-related spells and magic items.