Interesting question, but your answer will mostly be here

The One Being existed before realms were created along with the Elder Gods. It is an all powerful entity that the Elder Gods would hope never to have to face again. The Elder Gods managed to wage a successfull war versus the One Being and managed to shatter its consciousness into or using the Kamidogou. It is the One Being's consciousness which has created the realms and everything within them. You could therefore view all existence (minus the Elder Gods and possibly lesser gods) in the MK world as a dream of the One Being's, as it's consciousness makes up the very fabric that is the realms themselves.

The whole point of the MKD story is to prevent the fusion of the Kamidogou. Fusing them would mean putting back together the One Being's consciousness as a whole, therefore destroying the realms and all existence and awakening the One Being once again.

I'm pretty sure _Crow_ has some pretty interesting ideas about The One Being and how it relates to a theory of six realms...

I heard rumors early on that Raiden was the one being, but those can't be true now....

Maybe it's someone unexpected, like Mokap......