—Onaga creating the One Kamidogu in Tanya's ending in Mortal Kombat: Deception.
The One Kamidogu is an extremely powerful artifact produced after successfully fusing the six Kamidogu of Earthrealm, Netherrealm, Chaosrealm, Outworld, Orderrealm, and Edenia with the help of the Amulet of Shinnok.
The artifact was only mentioned and seen in Mortal Kombat: Deception, being the entire basis of the game. The artifact is mentioned indirectly once in the end of Mortal Kombat: Deceptions's Konquest, being described as "the means of attaining ultimate mastery over reality" by Onaga. The name of the artifact is officially referred to as the One Kamidogu in Tanya's ending of the same game.
In said ending, Tanya helps Onaga by finding the ancient texts that describe the process as to how to fuse the Kamidogu. Onaga succeeds in creating this artifact, but later on, when distracted and reveling in his victory, Tanya snatches the powerful item from the Dragon King and, therefore, is bestowed god-like power, which she uses to destroy Onaga and become Ultimate Overlord of the Realms. In Onaga's secret ending, he uses the One Kamidogu to merge the realms and destroy the Elder Gods.
It is important to note that the artifact was never created in the Mortal Kombat plot/storyline, only in these characters' special endings. After Shujinko destroyed the six Kamidogu, it made creating the One Kamidogu impossible.