- Bio Kard: "Centuries ago, before the rule of Shao Kahn, Outworld was controlled by Onaga, the Dragon King. Recently Onaga has found his way back to Outworld and is attempting to reclaim his former empire by any means necessary."
- Armageddon: "I had nearly regained my throne as Emperor of Outworld when my plans were undone. My pawn Shujinko had grown more powerful than I ever anticipated. Rallying warriors to his cause, he absorbed their fighting abilities and shattered the six Kamidogu, rendering me vulnerable. Only the Amulet I had retrieved from the sorcerer Quan Chi gave me protection. But as Shujinko landed his final blow, an outside force simultaneously ensnared my soul and expelled me from my host body. I found myself bound to a rune in the Netherrealm. I languished there until I was found by Shinnok, a fallen Elder God. Like me, he was trapped in that accursed place, denied domination of the realms by lesser beings. Shinnok offered me a chance at revenge: I would regain my rightful place as ruler of Outworld in return for my unquestioned servitude. I bow to no one, but I considered his offer. Shinnok was nearing his ascension from the Netherrealm, but he feared another defeat -- either by his enemies or his allies. He would emerge unchallenged if they were all eliminated. To achieve this, he needed an Edenian half-god named Daegon to defeat a fire elemental. The godlike energy Daegon would receive from that victory would bring about this slaughter. However, many other warriors also sought that power. Should Daegon fail, should some other warrior defeat Blaze, Shinnok's plans would go unfulfilled. Shinnok was most concerned with Shao Kahn, my most hated enemy. He proposed that I feign reluctance, but join Quan Chi's new alliance in order to stay close to the traitor. I would then follow him to the final battleground and prevent him from reaching Blaze. When Daegon finally won the prize, the kombatants would all be slain and I would be given Outworld to rule once more. As an added incentive, Shujinko, languishing in Shao Kahn's dungeon, would be mine to do with as I pleased. I did not trust this fallen Elder God. I suspected treachery. If all went as he had planned, I would most likely be among the dead. But I accepted his offer in order to free myself from that accursed Netherrealm. Satisfied, Shinnok conjured a portal and spoke through it. He then raised his hands and clenched his fists. With a demonic shout, my soul was freed from the binding symbol and I was released from the Netherrealm. I awoke in my original body, covered in dragon's blood. Before me stood Shang Tsung, Quan Chi, and my hated enemy, Shao Kahn. I will pose as Shinnok's pawn and join Quan Chi's alliance. Shao Kahn will never reach Blaze. But I would be a fool to allow Daegon victory. Once I have slain the fire elemental and have taken the prize for myself, I will have Shinnok's head. Onaga serves NO ONE!"
A millennium ago, Onaga, also known as the Dragon King, was the true ruler of Outworld. Hoping to gain eternal life, he ordered the last known dragon egg to be kept, hoping that the blood of an infant dragon would grant him immortality. However, he died before this could be fulfilled. Shao Kahn, one of his chief advisers, had him poisoned and took over his rule of Outworld. His army, however, was kept in a mummified state. This army was considered undefeatable because Onaga possessed the innate power to raise the dead. By means of constant resurrection during battles, his army truly was undefeatable. During the millennia that followed, Onaga was apparently forgotten.
Quite some time before Liu Kang's victory over Goro and Shang Tsung at the Shaolin Mortal Kombat Tournament, Onaga communicated through death to Shujinko through an avatar named Damashi. He persuaded Shujinko to embark on a quest to unite the Kamidogu, claiming he was carrying out the will of the Elder Gods, as well as granting him the power to learn the fighting techniques of warriors he encountered. Shujinko readily embarked on his quest, beginning to set in motion events that Onaga hoped would lead to his eventual return to power. Additionally, a sect of holy men still dedicated to him had been guarding the egg of the last great dragon in a chamber of molten lava. During the events of Mortal Kombat II, they captured the Outworld elemental Blaze, who was forced to guard the egg until it was ready to hatch and revive the king.
A few years later, The vampire Nitara manipulated Reptile to help her access the chamber of the last dragon egg. The chamber also held an orb that would separate Outworld from the vampire realm. After she destroyed the orb, Nitara left, but Reptile, feeling betrayed, arrived almost as soon as she transported herself to her native realm. At that moment, the egg hatched and sent its energy into Reptile, which fulfilled the prophecy of Onaga's return. The Dragon King, as such, used the transformed body of Reptile as a host, after he had gained the ability to rise himself from the dead through Shujinko's actions.
Meanwhile, Quan Chi and Shang Tsung, the Deadly Alliance, used their powers to resurrect Onaga's army. Their plan was to use this army to conquer Outworld and eventually, all the other realms as well. Raiden embarked on a mission with the Earthrealm warriors to Outworld to stop their plans, but his attempt ultimately fail, later on. His comrades were slain and he himself was defeated by the Deadly Alliance in person. However, their victory was short-lived. Just as his army stood ready to receive commands from Quan Chi, who had emerged victorious in a battle with his former ally Shang Tsung, Onaga returned. Even the combined might of Shang Tsung, Quan Chi and Raiden was not sufficient to defeat him. Raiden's desperate last measure was to use a self-destruct magic that released his godly essence, but that only succeeded in destroying Onaga's army and, presumably, the two sorcerers. Onaga was unaffected due to being resistant to all forms of magic and was now in possession of the amulet Quan Chi previously held.
It was at this time in which Shujinko, finally realizing he had been duped, took action. He traveled through the realms in order to find recruits to fight Onaga. All of these warriors came from extremely diverse origins but Shujinko planned to unite them to fight this enormous threat, who was now in possession of the six Kamidogu (found in the Temple background) and attempting to unite all of them into one.
Unbeknownst to Onaga, Shujinko had united many warriors and absorbed their fighting abilities, and was planning an attack on the Dragon King's stronghold. Shujinko had destroyed each of the Kamidogu, giving him the opportunity to eliminate Onaga. But as Shujinko landed the final blow, Onaga's soul was forced from Reptile's body and dragged down to the Netherrealm, where he was bound by Nightwolf. He remained there for a long time until he was discovered by Shinnok, who had also been trapped there. The fallen Elder God proposed that he would give Onaga his place at the Outworld throne in exchange for his help in returning his own power. Onaga reluctantly agreed, for the time being.
Shinnok informed Onaga about Blaze and the godlike power that would be given upon whoever defeated him in kombat. He proposed that the Dragon King join with Quan Chi, Shang Tsung, and Shao Kahn so that he may stand a better chance at attaining the power. Onaga did not trust Shinnok, and suspected that the former Elder God had something else up his sleeve. But with no other alternative to escape the Netherrealm, he accepted. Satisfied, Shinnok opened a portal to Outworld and sent Onaga back through, where he met with Quan Chi, Shang Tsung, and Shao Kahn and allied with them. He vowed that he would take Blaze's prize for himself, and would destroy everyone who ever stood in his way.
Onaga then fights on the Forces of Darkness during the battle of Armageddon. However, Onaga's desire for revenge against Shao Kahn proved to be greater than his desire to gain Blaze's prize. When Shao Kahn was knocked towards the edge of the Pyramid of Argus, Onaga grabbed him and flew far away from the pyramid. His former adviser managed to escape and return to the battle, and Onaga was killed at some point during the battle.
- Deception (Non-Canonical): "My enemies had failed to prevent me from fusing all the weapons of the Elder Gods into one. I used this ultimate weapon to warp reality, merging the realms into a singular existence. The Elder Gods were helpless, and I used their own power against them, in the void beyond the one realm. Not satisfied with merely ruling the one realm, I seized all that exists and merged it into myself. There can be only one consciousness. I am everything. I have become... The One Being."
- Armageddon (Non-Canonical): "The forces of Darkness defeated Blaze, and the Dragon King seized the prize for himself, attaining ultimate power. He immediately focused his wrath upon the one being he despised most: Shao Kahn. Long ago, Shao Kahn had stolen Outworld from him. Now Onaga would repay that treachery. Shao Kahn was beaten to the point of death, but Onaga would not see him die so quickly. He ordered Quan Chi and Shang Tsung to deal with the former emperor. While Onaga reclaimed his throne, Shao Kahn remained a captive in his own dungeon, tortured by those who had once pledged to him their allegiance."