These are the typical people/races/sapient species that are most often encountered in the underworld.
Humans – The most common people around. Weak, breeds like rabbits, very susceptible to magical corruption and influence. Which is pretty much why all the more powerful cultures and beings keep them around. Humans make wonderful raw materials for so many things. Not to mention they can breed with more or less anything.
Elves – Near immortal beings that use alchemy and magic to reproduce. Devoted to the study of perfection in all things, but in a very practical way. Elves believe that any situation or problem, given a set of priorities and resources, has one perfect solution. So each elf studies and practices to learn that perfect solution. Example: Not just mastery of fighting with a sword, but mastery of fighting with a sword against two humans who are brother and sister. Which is different from fighting with a sword against two humans that are brothers. Perfection is just a matter of learning the perfect response/action to any situation down to the smallest most precise detail.
Dragonborn – Humanoid dragons. Wings, breath weapon, a bit bigger than humans. They like rough hewn caves and are expert blacksmiths and jewelers. Have a good amount of dragon greed but unlike dragons they are perfectly happy to work together and trade for their wealth. Have basic thermal vision that allows them to see nearby heat.
Tieflings – Beings whose human ancestors were corrupted (or outright slept with) demonic forces. The corruption has been passed down and Tieflings are physically twisted and mutated by the demonic forces inside their bloodline.
Aasimar – Angels that gave up their immortality and power to become mortal champions of good (however the angel defined it). Appear as humans with near perfect appearance. They always try to do the right thing in any situation. When they die they are soon reincarnated. Each reincarnation has vague memories and can manifest skills from previous lives. Also often under the influence of their original angelic self that can take control of them at times.
Beastkin – Cat people, dog people, various other anthropomorphic animals in various mixes of the human/animal spectrum. Most are wanderers, scavengers, and gypsies. They aren’t really actively shunned but not exactly treated in the most friendly of manners. Even though they are their own races at this point, their mix of human and animal features reminds most people of magical mutations. Does not include Lizardfolk.
Lizardfolk – Humanoid crocodile people. One of the only true natives to the underworld near the inner sun. Though many are descended from ancestors that went to the surface and then were exiled back to the underworld like everyone else. They practice Primal necromancy and have a ancient tribal culture. Do not anger Lizardfolk in their own territory. Just don’t. That mistake is rarely survivable.