THE SAGE REALM
Also known as the Taoist World or the Demon Realm, it's one of the three realms alongside the Human and Heavenly Realms. It is populated by various creatures known as demons.
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In the Sage Realm, time moves slowly: one hour is equal to one day in the Human Realm.
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The gods split the universe into three worlds after the demons invaded the human race 5.67 billion years ago. While the Sage Realm can only interact with the Human Realm through specific portals or gates, it's closer to the Heavenly Realm and can travel there through the Skyscraper.
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Since the Sage Realm is close to the Heavenly Realm, displaying excessive power in the Sage Realm would draw the gods' attention. [1]
Oraeguk
Also known as the Ore Country, it's home to various monsters. It was destroyed during the Second Heavenly War by the gods and used to be ruled by the Demon Kings.
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During Mihu's reign as the fake Monkey King, Mount Hwagwa was in disorder. Its residents would be mind controlled, their main source of food was being plowed out and the relationship with their ally, King Uma, was getting worse. [2]
Devil monkeys live in Mount Hwagwa, a region in Ore Country, where Jecheondaeseong once served as their ruler.
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Their ally, King Uma, dwells in the Ox Demon King's Castle served by demon oxen.
The Skyscraper
The Skyscraper is an inverted building built within a planet in the Sage Realm. It's made up of 10,000 floors, with the last 7 being revealed to be the actual Heavenly Realm, although the entire Skyscraper is considered to be the gods' property.
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The gate to the Heavenly Realm is guarded by The Gatekeeper, which many titans have failed to defeat and beating him ensures a position as an Apostle in the Heavenly Realm. [3]
References
[1] Chapter 125
[2] Chapter 128
[3] Chapter 212