Onaga's Heart also called Heart of Onaga, also known as the Dragon King's Heart or Heart of the Dragon King, is an enchanted heart organ that belonged to the legendary Dragon King of Outworld himself, Onaga. This heart, and so too Onaga himself, possesses the power to raise the dead. It made its debut and first appearance being mentioned in the endings of Havik and Kabal in Mortal Kombat: Deception.
It is the true source of power behind Onaga's undefeatable army. For unknown reasons, his heart is what allowed Onaga to constantly resurrect his armies during battle.
Havik's Deception Ending
The kombatants had defeated the Dragon King, but left his broken body unattended on the floor of his throne room. Not long ago, a similar fate had befallen his former advisor and most hated enemy, Shao Kahn. Havik the Cleric of Chaos ripped the still-warm heart from the carcass and consumed it, thus absorbing Onaga's power to reanimate the dead.
Had the Dragon King succeeded in his plans for total domination, the never-ending turmoil of life would have come to a stifling halt. Those who defeated him believed that the realms were at rest once more, but Havik vowed to restore the Chaos that once ravaged Outworld by bringing the heroes' worst enemy back to life. Shao Kahn would rule again.
Kabal's Deception Ending
Havik had given Kabal's new Black Dragon recruits a task: Lure the heroes away from the Dragon King's corpse while he somehow retrieved the heart, and with it, Onaga's power to revive the dead. Apparently Onaga's ancient army had only been invincible by means of constant resurrection during battles.
The power to raise the dead would indeed prove quite very useful to the Black Dragon clan. Kabal was clever when saw the opportunity at hand: he quickly slew Havik and took the Dragon King's heart for himself. Havik was most impressed as he is somewhat smiling at him before he died.