- "It leads to a second human civilization that diverged from the surface of Eleg so long ago that nobody up here even knows about it."
- ―Voreese, in reference to the pit beneath Warrenhold
The Undercrust is a land of caverns deep in Eleg's mantle, which is inhabited by humans and high-temperature-biology humanoids.
Stucture[]
In Eleg's mantle there are a number caves, small enough that modern seismological sensors can barely detect them but large enough to house entire cities. Little has been explained about how the area is habitable, but they are known to have a lot of technology specifically designed to protect them from the heat. The Undercrust has a much, much larger temperature spectrum compared to the surface, beginning around five hundred degrees and ending around four thousand.
According to Voreese, both Sermung and Sai-hee spend much of their time in the undercrust, and each of them and their strongest supporters projects a large safe area, although the process this works by is unknown.
Population[]
According to Garovel, the Undercrust has a population of about 500 million people, although it is unknown how accurate his information was given that he hadn't been to the Undercrust in about 300 years. There are several races that live in the Undercrust. An example is the Hun’Kui(People of Ash), Bald Grey skinned people, with glowing white eyes without irises or pupils, they have no hair on their body. The average Hun’Kui can comfortably withstand temperatures of up to eight hundred degrees. If they left the Undercrust for the surface, they would instantly freeze to death.
Connection with the Surface[]
It is very difficult for normal humans and most servants to travel between the surface and the Undercrust. Few servants have powers that would let them effortlessly penetrate the sheer amount of rock between them, and any servant will also need to have enough passive Soul Defense to resist the extreme temperatures. There are a number of places where tunnels exist between the surface and the Undercrust, but the only one to have been revealed is the pit under Warrenhold, for which it was built where it was.