From publisher blurb:
Old-School Essentials is an adventure role-playing game of exploration, danger, monsters, and magic. The game is intentionally rules-light, putting the focus of play on imagination, improvisation, and fun. The rules are optimised for ease of use at the table, with meticulous attention to wording and layout clarity. What's more, Old-School Essentials is 100% compatible with the classic Basic/Expert game from 1981, meaning that decades of adventure are at your fingertips!
Classic Fantasy: Monsters
Old-School Essentials is a modular game where the core rules of the game are extended by additional "rules modules" which add content for different genres of fantastic adventure. This book contains a classic selection of monsters suitable for Classic Fantasy campaigns:
Over 200 fearsome monsters to challenge characters of all levels.
Complete encounter tables for dungeon levels 1–8+ and all types of wilderness terrain.
Procedures for generating parties of NPC adventurers and stronghold patrols.