Most of the Underworld is likely to consist of "typical" cavern and excavated areas, the type of terrain one would expect to find in a dungeon crawler: dry to damp passageways, small open areas with all sorts of stalactites and stalagmites, carved out chambers, stairwells and ancient ruins. We can think of this type of terrain as the standard for the Underworld, on which the rest of the biomes will be based.

As such, the typical cave biome will also hold the most variety of life. It's an easy place to get by, after all. It has water, but isn't too wet. It's cool to warm. It has a varying level of natural light (bioluminescence). And lots of room for different varieties of plant, fungus and creature to find a niche and thrive.

Variety breeds stability. We can expect the typical cave biome to have a much more solid foundation to its ecology than other biomes. If one food source dies out or leaves, others can be found with little stress to the system. If a predator starts to become numerous we can expect their predators to whittle down the numbers fairly quickly. Of all the biomes, the natural caves are the most balanced and stable.

The hot caverns biome requires a bit more specialization from its inhabitants. Filled with lava flows and pools, the hot caverns are quite dry and, well, pretty hot. For life to exist here it's going to have to get creative. Plants or fungus with deep roots to reach as far as possible for moisture; predators who can hunt at a distance from the cooler perimeter away from the lava.

This being a fantasy world, however, means this creativity can take some pretty dramatic directions. Hellhounds and lavabats, for instance, literally have fire in their bellies, with the later actually using lava as a means for digestion. These guys don't give a whit about the existence of water and thus would be some of the more common creatures found in this biome.

Light will be pervasive in the hot cavern biome, as perpetual as the flow of lava itself. How do predators work in a setting with almost no darkness? What changes in behavior will this cause? Creatures with a natural camouflage might become dominant, like that of the Shadow Beast who can blend perfectly into his environment regardless of the level of light.

We can imagine the Underswamp biome being almost the polar opposite of the hot cavern biome. Here, water is everywhere, and darkness and shadow. Finding sustenance is easy in the Underswamp and therefore is a less important skill to have. Being able to defend against your predators starts to become paramount. And in the Underswamp there are plenty of predators...

With so much food to be found growing in the Underswamp, from lichens and plants to trees and mushrooms, it is here we start to see the first true herbivores and grazers in the Underworld. But, unlike deer or antelope, grazers have to use more than simple speed to avoid becoming someone else's lunch; though big, the Underswamp isn't so big that a creature could run for miles to get away from a predator.

Size is always a good defense mechanism, and in the Underswamp we begin to see some truly massive animals, the lurker being a prime example. Other defenses come to light as well, the spiked shell of the stygian turtle, or the mind-numbing gaze of the catoblepas.

With so many things eating them, plants in the Underswamp have developed defenses too. Poisonous mushrooms are a constant danger, of course. Another is gasmoss, globular plants that fill themselves with swamp gas and explode when bitten into. The Underswamp is filled with unpleasant surprises.
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