Volcanic Fields

Magma can sometimes leap out of fissures in the earth that form with little to no warning, spitting flaming death up into the air. Every 100 feet of travel in a volcanic field, roll 1d20. On a 20, the terrain rumbles and spits up lava from a point within 50 feet. When a creature comes within 20 feet of the lava or starts its turn within 20 feet of the lava, it takes 4 (1d8) fire damage and makes a DC 10 Constitution saving throw. On a failure, it gains one level of exhaustion.