From publisher blurb: Welcome to the underground. Home to millions of dwarves, gnomes, halflings, goblins, and kobolds, these subterranean caverns are bustling with life, treasure, and glorious adventure. Explore the Cavern Sprawl: a cavern network most abundant with people, rare gemstones, new metals, and quests. Use new rules to prospect and dig for materials. Journey across the Umberhollow and steel your mind against the persistent psychic Thrum that threatens to transform you into its thrall. Reach the Edgemantle: the deepest area of the planet and forge epikorum, the strongest of metals. You'll find the following within the compendium's 28 pages: 3 underground areas detailed with features, treasures, cities, monsters, encounters, quests, and places of interest. 9 interesting metals, minerals, and gems 1 epic campaign quest hook centered around 5 gems of the Belt of the Primordials 3 subraces for a dwarf, halfling, or gnome that hails from the area. 1 Ranger subclass: embody the Stonewarden and use the power of the earth to protect others. 4 spells: from the flash crystal Crystallize to the destructive Tectonic Fracture 41 items: including custom gemstones, rock-cycling mauls, crystalline arcane prisms, and an ultra-destructive battleaxe. 5 monsters: from umbral thralls that stalk the Umberhollow to the massive psychic golem head that mysteriously waits there.