Overview
A realm where demons live. Seo Hweol and Hyeon Eum are known to have been in contact with this realm. The True Devil Realm is known to correspond to fire because of Karmic Fire. Originally, there were plenty of metal elements as well, but due to how long the Nether Devil True Lord controlled Karmic Fire, fire has a bigger proportion now.
History
The Blood Yin and True Devil Realm
In the distant past, when the True Devil Realm was whole, the Devil Realm was divided into two factions: the Demonic Devils and the Heavenly Fiends. Among them, the Heavenly Fiends would consume Demonic Devils as resources through their peculiar devilish arts. They were especially known for techniques that eroded the minds of Demonic Devils, corrupting and turning them into eternal slaves of the Heavenly Fiends, which was popular among the Heavenly Fiends at that time.
The Yuchok Tribe was once enslaved by the Heavenly Fiends, but one of their ancestors countered the mind erosion of the Heavenly Fiends by parasitizing the Heavenly Fiends' mind in reverse with clear self-awareness and personal standpoint, battling their mental power with a technique that became the origin of Mysterious Bizarre Gu.
Now, the worlds have split into the Blood Yin Realm where the Heavenly Fiends reside and our True Devil Realm where we Demonic Devils live. People from the Bright Cold Realm have no reason to see Heavenly Fiends... but it's a well-known fact in the True Devil Realm that the Heavenly Fiends have been peculiarly obsessed with the Bright Cold Realm for ages. Since you've come to the True Devil Realm, you might encounter Heavenly Fiends, so I'll give you this technique.
The Bright-Devil Great War
There was a great war 40,000 years ago called the Bright-Devil Great War, between the Bright Cold Realm and the True Devil Realm.
In the Bright-Devil Great War, the True Devil Realm was defeated by the Bright Cold Realm, and afterwards, it was said that the True Devil Realm had to cede some of its territories as colonies to various races of the Bright Cold Realm.
It was said that these races from the Bright Cold Realm would bring resources from these colonies in the Devil Realm to cultivate devilish arts for their cultivators.
Especially, the Human Race would buy devils from the Devil Realm as slaves and grind them into a pill called Devil Origin Pill, which was sought after by many races not only for reaching the Nascent Soul stage but also for cultivating in the Nascent Soul stage itself.
However, the Human Race, which made the pills, only provided a little rear support during the Bright-Devil Great War and could not get a share of the colonies when they were distributed. As a result, it seemed that the Human Race Grand Alliance decided to use the previous incident as an excuse to invade the Devil Realm and create colonies.