From the back of the book:
Expanding on the material presented in By This Axe I Hack, the By This Axe Companion includes 12 new Character Classes (4 warriors, 4 rogues, and 4 magicians), 6 new backgrounds for each, 72 new spells designed for the sword and sorcery genre, and 26 new special features to make your Character unique, plus a host of new rules and options, including new combat rules, weapon rules, wealth rules, wilderness rules, runeblade powers, and a collection of relics and artifacts for GMs who want to incorporate magic items into their games.
Although designed primary to supplement By This Axe I Hack and other roleplaying games based on the Black Hack, most of the material in this book is relatively OSR compatible and should be easy to modify for use with any version of the world’s most popular fantasy roleplaying game.
Based on David Black’s game-changing The Black Hack, By This Axe I Hack takes its cues from Robert E. Howard’s Conan the Barbarian and Kull of Atlantis, Roy Thomas and Barry Windsor Smith’s Red Sonja, Michael Moorcock’s Elric of Melnibone, and Fritz Leiber’s Fafhrd and the Gray Mouser.