First introduced in Mortal Kombat: Deception, The One Being is considered to be the origin of each of the six Realms of Mortal Kombat (Earthrealm, Netherrealm, Outworld, Orderrealm, Chaosrealm, and Edenia). According to ancient legend, in the beginning of time, there only existed the One Being and the Elder Gods. The One Being, in his foolish pride, declared war on the Elder Gods in attempt to gain ultimate power. Eventually, the Elder Gods defeated the One Being, and, in efforts to weaken the One Being's omnipotence, the Elder Gods separated the One Being's consciousness into six realms, each carrying a representative Kamidogu which held the essence of the realm. The legend decrees that if one were to obtain all six Kamidogu from each realm in combination with the Sacred Amulet, he or she would be granted ultimate power. In Deception, the warrior Shujinko undergoes this task through the guidance of Damashi, a spectre who, in fact, is a representation of the Dragon King, Onaga.

Because each realm is essentially a piece of the One Being's consciousness, the One Being flows through all life. Many of the warriors in Mortal Kombat are attuned to the voice of the One Being and, in turn, carry out his wishes in the One Being's attempt to regain his power. Examples of these warriors include Shujinko, Quan Chi, Reptile, and Onaga.

The only entity that is immune to the One Beings manipulation and influence is the fallen Elder God Shinnok.

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Storyline

At the time of the universe's creation, there were the One Being, and the Elder Gods. The One Being, arrogant as a result from its nigh-omnipotence, foolishly declared war on the Elder Gods. In order to defeat the One Being, the Elder Gods created the Kamidogu and with it, they shattered its consciousness, creating what is now called existence.

For aeons, the One Being's subconscious remained in a dormant state. It is unknown if the Dragon King was the first to serve the One Being's purposes, but because of Onaga's hunger for power, Outworld was created. After he died, Shao Kahn took control of the realm and continued his will by invading other realms and adding them to the Kingdom of Outworld. Through Shao Kahn's continued conquests, more of the One Being's consciousness would amass within the Outworld.

Shao Kahn and his forces were defeated during the invasion of Earthrealm, and thus dominance in Outworld dwindled; however, the One Being had other plans in the works as well. From beyond death, Onaga deceived Shujinko into traveling the realms and retrieving the shattered pieces of the Kamidogu. After a lifetime of searching, the Kamidogu were recovered and with it, Onaga's means of returning to the living. Onaga traveled to the Nexus to thank Shujinko and then, he returned to Outworld.

Quan Chi was now the super power in Outworld; with the help of Shang Tsung, Raiden was defeated and Quan then bested Tsung in Kombat. Onaga confronted Quan Chi and challenged his power - the sorcerer was no match for the Dragon King. Even as Shang Tsung and Raiden joined the battle, Onaga's power was too much. Onaga retrieved the artifact he sought and now holds the key to releasing the One Being.

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Speculation
The full extent of the One Being's manipulation is unknown, and it is unsure whether his plan had succeeded. The general consensus with fans, however, is that it had indeed succeeded and, as a result, the One Being will be the final boss, albeit possibly in a weakened state, in the next game of the series.

It is also rumored that the One Being, working through the undead priest Havik, resurrected Liu Kang's corpse to serve as a distraction along with the thunder god Raiden, who had grown angry after discovering the state Earthrealm was kept in by Mortal Men and the actions of Shujinko, who is solely responsible for resurrecting Onaga to keep any forces wishing to foil its plans from doing so. While it is a rumor, it would seemingly sound convincing, especially when Havik is one who thrives on chaos and carnage.