HUMANS
They are the main inhabitants of the Human Realm. They are the least powerful of the three species, with only one in a thousand years possessing strength on par with a god, Jin Taejin being one of such humans.
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They were constrained by the Taboo, making them unable to attack the gods, and have long served as a source of life energy for the gods to maintain their youth.
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Every now and then, Okhwang would turn on the "Ark" to reverse humanity's advancement in order to keep them under control or to steal human bioenergy . Some examples of such events are the Noah's Ark, Sodom and Gomorrah, The Tower of Babel and The Black Plague. [1]
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After Ragnarok, Ultio R lifted the Taboo and the emergence of Generation X humans followed.
The Taboo
When the three species coexisted for many millennia, the demons tried to subjugate humans. One of the leaders at the time, Ungnyeo, asked the gods for help and they agreed to lend humanity their powers, resulting in the first known instance of borrowed power being directly infused into human bodies. The people using it either grew older or younger, which was a high price to pay for using this form of borrowed power.
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After the war, the god Hwanung fell in love with Ungnyeo and decided to destroy the system, allowing humanity to be free of the aftereffects of borrowed power. Hwanung was brutally executed, and as a result, the three species were divided into different realms, with Okhwang remaining in the Human Realm to keep the humans in check and prevent them from abusing the newer form of the borrowed power system. Afraid of humanity's potential, the gods placed the Taboo on humanity. [1]
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Borrowing power became commonplace and human abilities were enhanced after Ultio R broke the Taboo. However, a remnant of it persisted in their DNA, and as a result, a new generation of humans who were unconstrained by it was born.
Generation X
These new humans don't need to borrow power because they have their own exclusive abilities. Power Transfer is a common ability that allows them to impart not only their skills to one another, but also their senses. They are later revealed to be Prophets, but were stated to be a little bit different from them. [4][5]
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For instance, Dean, the first introduced member of Gen X, was able to mimic both borrowed power and martial arts, a separate ability from his actual borrowed power, Verdandi. Through the course of the narrative, he was also given the special abilities of four other Gen X members through Power Transfer.
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The X Syndrome is a rare disease found in the generations born after Ragnarok. It appears around age five or six and increasing in teenagers. Its symptoms are high fever, paralysis and blood cancer. Such patients only have a 30% survival rate and only 5% of them are actually Generation X. [2][3]
References
[1] Chapter 255
[2] Chapter 366
[3] Chapter 411
[4] Chapter 558
[5] Chapter 473