The plane of the underworld does indeed have an incomprehensibly vast surface, of two biomes, a jungle and a desert, this particular demon is the one that dwells in the jungle, they can look a lot more varied than this example, but this is the most common, they often will come to the mortal realm to rule over areas of wilderness, placing it under their protection, they can manipulate plantlife to their advantage.

They bleed an orange, sticky sap that's sweet.

EDIT: Almost forgot, the bottom of the rivers in this jungle-like area are lined with the hands of lost souls, they will try to grab at any living being, and drag them down so they may join the mass.