Four Against The Netherworld is a supplement for Four Against the Darkness, for character levels 6-9.
In its pages you will find:
New classes: Demonologist, Cambion, Cleric of Zur, Cleric of Foryos, and a reworked Paladin;
A complete economic system for the Netherworld;
Rules for hex-based exploration, with plenty of terrain types, and a downloadable color map of the known regions of the Netherworld;
Six new spells for wizards and cambions;
New Reactions: Burn Souls, Capture, Conversion, Demonic Pact, Mercenaries, Demand Obeisance, and more;
Tables to generate encounters and events;
Plenty of new, deadly monsters, including randomly generated Lovecraftian aberrations.