|the jagged brimstone hisses and spits ominously as it crunches under your feet.|
Brimstone is an encrustation of sulfur around volcanic vents, and it has strong associations with divine wrath and Miltonic hell. Lakes of brimstone appear in the deeper levels (never before level 18) and exists in three phases: inert, active, and burning.
When brimstone is active, it can be lit by neighboring fires or by firebolts -- it behaves like any other flammable terrain -- but it can also burst into fire spontaneously. After the fire burns down, inert brimstone is left behind which will spontaneously, and invisibly, become active again.
For photos of a real lake of burning sulfur, see The Boston Globe's beautiful photo series, Kawah Ijen by night.