Nezuthrope

The Nezuthrope, or more often simply Nezu, are a sapient, hair-covered bipedal race with long tails and elongated faces. They are similar enough to rats, that many would mistakenly call them ratmen or worse assume they are one of the shapeshifting peoples. The Nezu have much more in common with rats than they do with other humanoids, so it would be more accurate to think of them as an evolution of the rat. They share many of the preferences as their diminutive ancestors but have a well-developed language that uses body posturing as well as auditory and olfactory cues. From birth, they live within a caste system, but this is somewhat fluid. Those that do well or curry the right favors can be elevated the same as those that betray their clan or are caught, or accused, of wrongdoings will have their rank stripped.
On Ruathawaul they reside entirely within the body of the planet itself, in the regions commonly called the labyrinth. They are the only people believed to be native to the planet and it is uncertain how they survived its destruction. After the Alvar terraformers reconstructed the planet from the asteroid field they discovered, swarms of rats troubled the refugees fleeing the Skyfall event. Rats that the Alvar did not place on a planet that should have only been seeded with what the Alvar chose. While the Alvar did place a variety of rodents out of a sense of completeness, it simply should not have been possible for them to breed at that rate. By the time the source of the rats was discovered it was too late to prevent a conflict that would have far-reaching and long-lasting consequences.
In their haste, the Alvars' reconstruction of Ruathawaul was nearly perfect, but insufficient time was spent researching the ruins and translating the ancient scripts. It was not until after the Guild responsible for engaging the Nezu in a conflict instead of diplomacy did enough of the scripts get translated. It is believed that the Nezu were contracted to be a defense against invasion by the prior Lords of Bloheron. By engaging in a direct attack against the Nezu they are obligated by that contract to eject the invaders from Ruathawaul. The Alvar punished the hasty leaders of the offending guild by tasking them to be the eternal enemy of the Nezu. It is their hope the Nezu are able to identify those people as separate from the rest. And so far there have been no direct deaths between the Nezu and people not of that guild. The few times Nezu have appeared on either of the surfaces of Ruathawaul they engaged mostly with that guild. The fighting is particularly vicious in outer surface engagements. They seem generally disinterested in the goings-on of the interior.
While the Alvar do not have a complete narrative about the contract which binds the Nezu, it is known there is some sort of promise that was made upon the completion of their service which would end that contract. The planet they were supposed to protect was shattered into rocky debris and the original owner of the contract was long gone possibly for millennia. Alvar research into it ceased after the second revolt on Muaghtykus during an attempt to repopulate it following Skyfall. Once again, the Alvar retired from actively guiding the system and gave the races the self determination they demanded. It didn't really matter if the Nezu eventually chased everyone out of and off Ruathawaul. Skyfall was over, most of the population was saved and they all originally came from other planets; their ancestors at least. They could go back or fight to stay on Ruathawaul themselves.
Other sapient races and non-sapient monsters which might prefer the labyrinth for their homes have to vie with the Nezu for space. Even if there is no conflict in the beginning when they settle the subterranean spaces, eventually the Nezu find them and try to expel their settlements. This increases the conflict with surface peoples, not with the Nezu, but from those they displace. A new guild was founded whose members patrol the labyrinth and the areas of openings to it. Their goal is to control and manage overall monster populations within the labyrinth to prevent sudden massive incursions near settled areas. Only the highest ranking members are powerful enough to delve deeply to Nezu colonies where interactions, often tense and unsure, with the Nezu can happen. In this way the guild has been able to learn about the Nezu and try to complete the research the Alvar abandoned.
They have learned the Nezu are an intelligent race and they have been able to learn some of the surface languages. Some very limited trade has happened and a few Nezu have acted as guides within the labyrinth. Explorers from the guild have also learned that the labyrinth also creates life. That which is killed or otherwise in the labyrinth can be reproduced by some generative means. There is too much of a language barrier for the Nezu to explain how it happens or even if they know for certain. Explorers have witnessed fissures in the rock forming into archways from which a green vapor and soft organic masses tumble and shape themselves into all manner of creatures in seconds, fully grown. Whether the Nezu Contract has anything to do with it is conjecture. It is believed the green vapor seen in Nezu colonies and the green vapor from the spawns are connected as are the green crystals the Nezu carry and are sometimes seen to consume. Explorers also report that similar green crystals can be found inside the bodies of spawn which do not have the same entrails as creatures born on the surface. Nezu will trade earnestly for the green crystals and in that way deep explorers can sometimes work out a deal for supplies to extend their time in the labyrinth.