The Caverns of Cynder is a hostile Alien World, deep underground and overflowing with lakes of molten magma! There is some alien architecture such as pathways and ductwork to funnel the lava in practical and ceremonial ways. In the Caverns, Dark Stone is melted into the magma itself and can be found in liquid form! This has led some industrious explorers, both human and otherwise, to set up mining equipment in an attempt to siphon it out of the rock itself. Of course the Heroes must always beware of the foul creatures lurking the corridors as well as those that will rise up out of the molten rock itself to attack — not to mention the constant threat of eruptions and rising lava pools at your feet!

Shadows of Brimstone: Caverns of Cynder Expansion offers a new Otherworld for players to explore, with a set of 18 new double-sided map tiles. There are new Encounter cards, artifacts, Otherworld threats, and map cards, as well as three of the new lava men Enemy.

Cynder Tiles include a mechanism in which you can take damage if you pass over certain fiery spaces. If you have a high Agility you get to roll two dice, so swift and nimble characters are less likely to burn. Cynder also has mechanisms in which Clothing items make you susceptible to heat exhaustion and burning, so there's some interesting decisions to be made about how many items to keep equipped from turn to turn.