Demon World

Upper Abyss
The ground was scorched black soil, the forest was twisted black, and a large river formed by lava flowed slowly. The momentum moving the river came from the dozens of waterfalls of magma from the cave walls. They flowed from a great height accompanied by green smoke and sparks, stirring the airflow of the underground world, making the air hot. The hot winds rolled the black soot up from the ground, causing the air to be pretty dirty. There was also a range of mountains, some of which were high and low, but most of them had the same color and tone. It could be said that this was a black and red world.
 * Black was the black ashes, while red was the redness from the flames