For eons, 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.
It also said Onaga was the one being in his ending. (Which you have to unlock it even states this) See his ending if you don't believe me. When the Elder Gods shattered the Kamidogu which gave Onaga the One Being his power, he awoke and the realms were created, he then became ruler of Outworld. Until Shao Khan wanted his power and poisoned him but not even death could stop Onaga. *His soul didn't go to the Nettherrealm also proving this correct.
And also that part is a little off Onaga is the one being who is the One Beings (sub-counciousness). You see when the One being was the only thing around with the Elder Gods they became bored and created the Kamidogu and broke them to shatter him sleep, thus he came back in the form of Onaga.
You see why the hell in Li Mei's ending would he join the Kamidogu and then become the one being, with Li Mei at his side. He wouldn't, since the One Being is one character as you said. The awake form of him is Onaga.
The sleeping form is the One Being which can only happen when the Kamidogu and the Sacred Amulet are joined to be sent to the Elder Gods, but it is also the key to fusing them together.
Also didn't you say Blaze in one forum was the One Being? I mean where is proof of Blaze being such?
Also it doesn't explain why Raiden would attune to the One Being. Unles I misread Attuned means to put up with/become a friend with.
I don't think so. Raiden was pissed at Shujinko for releasing the Dragon King who IS the one being. Why else would he go dark over that on Shujinko? I mean sure Onaga is powerful, BUT Earth Realm has always prevailed. Even if most of Earth-Realms warriors died. They still succeeded in the end.
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.
Shao Khan was defeated during the invasion of Earth Realm? Uh MK3 was along time ago. You see Khan gave up before MK4 due to Liu kicking his ass if you don't believe me play MK

A the begining intro sort of states this. During MK2, and 3 Liu beat him but he didn't die.
After the events of MK

A everyone though Khan was dead but, it was a clone and he was alive. Thus his appearance in MK

.
Also I think you mean DA's forces. Khan was thought dead during MK

A and all of his followers joined the DA. Thus it couldn't have been Khan's forces unles you mean MK3 then see the part above this. Before the quote.
Now it will be explained whether or not Onaga is the One Being in MK:A BTW I'll have to check it again most of that I know but I'm pretty sure he is the one Being.
Edit-From what I've seen he could be, but most likely isn't from what I've read. Through the whole MK storyline :shock: I'm still trying to not barf LMAO.