From the Introduction:
The finely hewn and set stone walls have made way for natural rock, worn smooth over the years probably by countless denizens passing through. At the end of the corridor you can see the glow of molten rock and an almost unbearably hot wind smelling of sulphur and brimstone blows the way you have come from.
The forge of the primordials consists of a set of rock pillars, connected by narrow natural rock bridges, emerging from the magma below. A number of anvils is set up around a small magma pool at the center of the largest pillar with steel, brass and copper ingots piled between them. Otherworldly crystals glowing with an infernal light not unlike that of the molten rock below float at the center of each of the smaller columns surrounding the central one.
Beyond these pillars and the rock face with the cave entrance leading back up a sea of molten rock stretches as far as you can see, the hot air and fumes rising from the magma basin obscuring the other cavern walls, if there are any.