The One Being was first introduced in Mortal Kombat: Deception. According to the storyline, the One Being is considered to be the origin of all realms in the Mortal Kombat universe.
Long before the creation of the realms, only the One Being and the Elder Gods existed. In an effort to weaken its control over them, the Elder Gods joined together and defeated the One Being, separating its consciousness into realms - Earthrealm, Outworld, Edenia, Chaosrealm, Orderrealm, and the Netherrealm. Six Kamidogu were created to represent each realm, and each held a portion of the One Being's essence inside.
In Mortal Kombat: Deception, with the aid of Shujinko, Onaga was able to obtain the six Kamidogu artifacts, along with Shinnok's sacred Amulet. In doing this, it is believed that the One Being has now combined the fragments of its consciousness. In Ermac's ending, it states that he could sense the One Being's presence.
The future of the One Being's role in the Mortal Kombat universe is still uncertain at this point.