The realms under the surface of Kaelaross are the disputed territories of two great factions - the cold-bloods and the grey goblins.
The Coldbloods include lizardmen, troglodytes and the enigmatic snakemen of whom there are several subraces. including Rattlers.
They have many reptilian animals used as beast of burden, attack beasts and mounts, such as different lizards and some dinosaurs where the size of cavern permits.
The Grey Goblins are vicious and evil but also surprisingly well organised subrace related to the normal goblins found closer to the surface. They are noted for powerful shamans and warriors who can lead legions of goblin soldiers.
The Grey Goblin shamans are adept at using powerful chaotic magic to shape life forms into new hideous species.
There are both unintelligent animals and also intelligent creatures that are either solitary or form small bands.
Minotaurs, trolls and ogres are found underneath, and although all three races can be hired as mercenaries by both main factions, they tend to be used more by the grey goblins.
Dwarves and gnomes can both be found in the Underworld but do not parley with either faction - they have ample reasons to hate both the grey goblins and the cold-bloods. They have intermittent contact with civilization on the surface, but some gnome colonies remain isolated for decades.
The Underworld has its own ecosystem, at the base of which there are the fungi. There are many types of fungi, most of which are harmless but difficult for humanoids to digest. Some of them are poisonous if eaten, while a rare few are dangerous to even approach. The most well-known to adventurers is the shrieker, which makes a terrible noise when it senses approaching creatures, and the dreaded yellow mold, which emits deadly spores when disturbed. Fungi can be found throughout the Underworld, and in some large damp caverns they can form their own fungal forests.
The herbivores are not normally mammals - the creatures that feed on the fungi include cave locusts, giant fire beetles and subterranean iguanas.
The predators that feed on the herbivores are wide-ranging and usually dangerous to adventurers. They include the humanoids mentioned above, as well as many types of monsters and giant arthropods.