The Salamander's Cavern
After following a narrow, winding tunnel a mile or more, the party finds themselves in an air pocket.* Far beneath them flows the life-blood of the Mountain God—a river of magma.
Across a narrow bridge arching over the river is the dwelling place of an aged but cunning salamander. In a natural pool, magma bubbles. A steep ledge is where the creature sleeps.
The cavern is also home to three magmin, minions of the salamander. If approached/attacked, they will do whatever is necessary to herd their foes into either the pool or over the edge and into the molten river. The intense heat in the cavern makes it difficult for surface dwellers to breath and will weaken them physically.
*(As this is a grand cavern, the scale should be at least 1 ■ = 10'.)