Many ages ago, long before Shao Kahn became the emperor of Outworld, there was another ruler, known as the Dragon King. In hope of gaining immortality, Onaga believed that the blood of the infant dragon would grant him eternal life. He ordered for the last known dragon egg to be kept for his special feast, but before this task was completed, he was poisoned by his chief adviser, Shao Kahn. Onaga's army went into a mummified state, Shao Kahn became the ruler of Outworld, and Onaga was forgotten.
Centuries later, an orb of orange light appeared in Earthrealm, calling itself "Damashi". Disguising itself as a friend, this being communicated with a young boy named Shujinko. He told the boy that he were the chosen champion of the Elder Gods, and gave him the power to absorb the abilities of the warriors he would encounter on his journey. In reality, Onaga himself was hosting this avatar all along. Onaga deceived Shujinko into restoring the six Kamidogu, claiming that he was destined to carry out an all important quest for the elder gods.
As for the original egg that Onaga wished to devour, it was hidden away in a secret chamber located in Outworld. Nitara knew of this chamber, and convinced Reptile to help her access it. The chamber had an orb that would separate Outworld form Nitara's realm. She took the lethal lizard to the egg, and then destroyed the orb, leaving via teleportation and deserting Reptile. The egg hatched and sent its energy into the betrayed reptilian, and Onaga was reborn in a new body, therefore fulfilling the prophecy of the Dragon Kings return.
All the Kamidogu were collected, and Onaga would then reveal himself to the now elderly Shujinko. Onaga laughed at the old man for wasting his entire life. Onaga planned on combining the six Kamidogu, and after doing so, he would become an ancient creature older than time itself, known as The One Being. In other words, Onaga would become reality itself, and be able to destroy the Elder Gods, and the universe as we know it would cease to exist. Onaga only needed a final artifact to help him gain ultimate power, Shinnok's Amulet. The sorcerer Quan Chi was in possession of this powerful piece, and Onaga planned to take it for himself.
Quan Chi and Shang Tsung were nearly unstoppable, they defeated Lord Raiden in battle and fully resurrected the Dragon King's immortal army. Quan Chi then thought that he would not need his partner any longer, and strangled Shang Tsung nearly to death. Quan Chi planned to rule the realms himself with the power of the amulet. Little did he know that the Dragon King had returned. The mummified army of the Dragon King immediately knelt down when he entered the palace. Quan Chi watched in shock, not believing this could not be.
Quan Chi used his magic to try and stop the menace, but to no effect. Shang Tsung knew that Onaga's rule would literally mean the end of everything, and got back up on his feet to help his former best friend. The Deadly Alliance was once more, but their combined strength had no effect on Onaga, but Raiden got back up as well to help. Shang Tsung and Raiden give each other a knowing look, and a new alliance was formed out of desperation, but not even the combined might of the powerful warriors had even the slightest effect on Onaga. Raiden would then create a nuclear lighting blast, sacrificing himself as well as the two sorcerers. This would prove useless however, as Onaga simply shrugged it off and finally retrieved the amulet.
Fortunately, Shujinko used the mighty gift that which he received as a young boy from Onaga against the deceiver himself. In a final showdown, Shujinko confronted Onaga in his palace. Shujinko then destroyed the 6 Kamidogu, greatly weakening Onaga's strength. He then took his revenge upon the deceiver and ruthlessly beat the Dragon King to death. Redemption was his to claim.
Onaga appears to be the most powerful and feared being in the MK universe. The fact that Shao Kahn poisoned Onaga would mean than even he was afraid of the dragon kings power. Onaga's durability is incredible, being able to resit the combined might of Lord Raiden and the Deadly Alliance with visibly no damage at all. Raiden even sacrificed himself in vain trying to stop the Dragon King. When Onaga acquired the Kamidogu, he became virtually unstoppable in physical confrontation.Powers
Onaga has a considerable amount of magical powers. He appears to be immortal, as even though he was apparently long dead, he returned to the realm of the living as an astral avatar. He can breathe flames in concentrated balls or even as a stream of fire, regenerate lost limbs, raise the dead with the power of his own heart, and he can even fly too.