The Outworld Invasion of Earthrealm (or the Battle of Earthrealm as it is otherwise known) is a major event in the history of the Mortal Kombat video game franchise. This event occurs in Mortal Kombat 3 and later in the third arc of the Mortal Kombat (2011) story mode.
Following Outworld's loss to Earthrealm both in Mortal Kombat and the Outworld Tournament, Shao Kahn grows frustrated as he no longer has a claim to Earthrealm. However, he devises a scheme in which he will be able to force open the portals to Earth, and has Shang Tsung and his Shadow Priests carry out this plan. Shang Tsung is able to resurrect Sindel - former queen of Edenia and Kahn's bride - on Earthrealm, thus allowing Kahn to enter Earthrealm in order to retrieve her. However, Kahn forces the portals to remain open, allowing his armies to invade.
Upon entering Earthrealm, Shao Kahn begins to slowly merge Earth with Outworld: Outworld landmarks begin appearing across the planet and Kahn's fortress appears in the centre of New York City. Through Kahn's dark magic, Earthrealm was stripped of life, its inhabitants' souls taken as trophies. However, a handful of souls were spared thanks to the thunder god Raiden who bestowed divine protection upon a few chosen warriors. These warriors gathered together and ventured into New York, evading Kahn's extermination squads and various Outworld monsters in order to reach Shao Kahn's fortress. There, it was the Shaolin monk Liu Kang who battled Shao Kahn and ultimately defeated the emperor in Mortal Kombat. Kahn then retreated and called his forces back to Outworld, thus reversing the merger of the realms and restoring Earthrealm.
With Earthrealm restored to life, the population became all too aware of the threat posed by other realms. The US Government formed a new division of the Special Forces known as the Outerworld Investigation Agency which was tasked with contacting these alternate dimensions and assessing their threat potential in order to prevent another interrealm conflict.
In the alternate timeline triggered by Raiden's message to his past self, the Outworld incursion into Earthrealm was brought forth by the necromancer Quan Chi. The Netherrealm sorcerer, unrestricted by the rules of the Elder Gods, travelled to Earthrealm where he conducted the ritual to raise Sindel from the dead, thereby breaking her magical ward that stopped Shao Kahn from setting foot on Earth. The armies of Outworld poured through into Earthrealm where they fought against the humans' police and military forces in open warfare.
Raiden rallied the champions of Earthrealm to battle, this time fighting alongside them as they bravely struggled against Shao Kahn's forces. But as time went on, the Earthrealm heroes suffered heavy losses and Raiden sought the aid of the Elder Gods. The Elder Gods refused to intervene, declaring Kahn's invasion as legal according to the rules of Mortal Kombat.
Due to the machinations of Quan Chi and a ferocious assault by an empowered Sindel, many of the Earthrealm warriors were killed. In desperation, Raiden approached Quan Chi in the Netherrealm and attempted to strike a deal with him, offering the sorcerer his own immortal soul and the souls of the fallen champions in exchange for the Brotherhood of Shadow's help in defeating Shao Kahn. Quan Chi denied Raiden, stating that Shao Kahn had already promised the souls of Earth's heroes to him, but inadvertently assisted the thunder god by stating that the emperor would soon enter Earthrealm in order to carry out the merger with Outworld. With these words, Raiden finally understood the meaning of the message from his future self: "He must win".
Raiden returned to Earthrealm and informed the remaining heroes that Shao Kahn must be allowed to begin the merger of Earth and Outworld as doing so would be in violation of the Elder Gods' decree, thus giving them cause to intervene and punish Kahn for his transgression. Liu Kang was unconvinced and believed that Raiden had gone mad, opting to attack the thunder god. Raiden defended himself by casting lightning upon Liu Kang, but in doing so immolated the Shaolin fighter. Kang died in Raiden's arms, cursing him for supposedly bringing about Earthrealm's doom.
Shao Kahn then stepped through the portal and proceeded to beat Raiden to a pulp while the thunder god did nothing to defend himself. However, as the emperor began to conduct the merger of the realms, the Elder Gods at last responded, granting their power to Raiden who then fought Kahn in a final showdown. Raiden was victorious and Shao Kahn was destroyed by the Elder Gods. With their emperor slain, the armies of Outworld retreated back to their realm. Earthrealm had been saved, but at great cost. Most of its heroes had fallen in battle and the fabric of the realms had withered as a result of the attempted merger, making it possible for the fallen Elder God Shinnok to escape from the Netherrealm and wage war against Earth, Outworld and the Heavens themselves.
- Raiden - God of thunder and protector of Earthrealm. In the original timeline, he could not participate in battle due to the merger of the realms, but was able to ward the souls of a select few warriors who would fight to save Earth, meaning that Shao Kahn could not claim their souls as he had done with the rest of Earth's population. In the new timeline, Raiden fought beside his mortal allies and was responsible for killing Liu Kang as well as ultimately defeating Shao Kahn.
- Liu Kang - A Shaolin fighting monk and Grand Champion of Mortal Kombat. He was the one to defeat Shao Kahn in the original timeline, but in the new timeline he witnessed the deaths of most of his comrades, shattering his faith in the Elder Gods and Raiden. After hearing that Raiden intended to let Shao Kahn carry out the merger, he attacked Raiden only to be killed. His soul was claimed by Quan Chi who turned him into a revenant.
- Johnny Cage - A former Hollywood actor who fought alongside Liu Kang and Sonya Blade in Mortal Kombat. In the original timeline, he was killed by an Outworld extermination squad, but his soul could not pass on due to the merger of Earth and Outworld, thus he was restored to life when the merger was reversed. In the new timeline, he survived and he and Sonya Blade married years later.
- Sonya Blade - A lieutenant in the US Special Forces. She had tried to warn her superiors about the Outworld threat but her claims were dismissed as nonsense. She fought alongside the other chosen warriors and survived, later founding the Outerworld Investigation Agency following the Earth's restoration. In the new timeline, she and Johnny Cage became romantically involved and the two married. Sonya later gave birth to a daughter, Cassie Cage, who would join the Special Forces when she came of age.
- Kuai Liang - Better known as the ice ninja Sub-Zero. In the original timeline, he abandoned his clan, the Lin Kuei, in opposition to the Grandmaster's Cyber Initiative. In the new timeline, he was captured by the Lin Kuei when they had allied with Shao Kahn and was turned into a cyborg, but was later defeated in battle and his mind was restored. However, he would later be killed by Sindel and become one of Quan Chi's revenants.
- Scorpion - Formerly Hanzo Hisashi of the Shirai Ryu clan before being killed by Quan Chi and turned into an infernal revenant. In the original timeline, he had killed Kuai Liang's brother Bi-Han in Mortal Kombat and chose to atone for his murder by acting as a secret guardian for the younger Sub-Zero. In the new timeline, Quan Chi keeps Scorpion confined to the Netherrealm and he does not take part in the war.
- Kung Lao - A Shaolin monk and member of the White Lotus along with Liu Kang. He was killed by Shao Kahn in the original timeline only to return to life after the merger was reversed. In the new timeline, he did not take part in the war at all as he was killed during the Outworld Tournament. His soul was turned into one of Quan Chi's revenants.
- Jax Briggs - A major in the US Special Forces who helped Sonya found the Outerworld Investigation Agency in the original timeline. In the new timeline, he was killed by Sindel and Quan Chi turned him into a revenant.
- Kitana - Adopted daughter of Shao Kahn who betrayed the emperor after learning the truth behind her mother's death and the conquest of her homerealm Edenia. She sided with the heroes of Earthrealm and later rallied the remaining Edenians to wage war against Shao Kahn to separate their realm from Outworld. In the new timeline, however, Kitana is killed and she becomes one of Quan Chi's revenants.
- Smoke - A former member of the Lin Kuei. In the original timeline, he was forced to undergo cyberization and was later sent to hunt down Sub-Zero. In the new timeline, he escaped only for Sub-Zero to be captured in his place. Smoke is killed by Sindel and becomes a revenant.
- Jade - An Edenian warrior and close friend of Kitana. In the original timeline, she is forced by Shao Kahn to hunt down Kitana and bring her back to the emperor. In the new timeline, she joins the forces of Earthrealm only to be killed by Sindel.
- Kurtis Stryker - A New York riot cop who receives Raiden's protection. He survives the invasion in the original timeline, but he dies in the new timeline and becomes one of Quan Chi's revenants.
- Nightwolf - A native American shaman who receives Raiden's protection. He survives the invasion in the original timeline, but he dies in the new timeline, sacrificing himself to stop Sindel's rampage.
- Kabal - A former member of the Black Dragon crime syndicate who joined forces with Stryker's police squad. He survives the invasion in the original timeline, but he dies in the new timeline.
- Shao Kahn - Emperor of Outworld. In the original timeline, he carried out the merger of Outworld and Earthrealm only to be defeated and forced to retreat. In the new timeline, however, Kahn is punished by the Elder Gods for attempting to merge the realms without having won Mortal Kombat, and so the gods destroy him completely.
- Shang Tsung - Demonic sorcerer and former Grand Champion of Mortal Kombat. He was responsible for Sindel's resurrection in the original timeline, thus allowing the invasion to commence. In the new timeline, his role in reviving Sindel is taken by Quan Chi and Shang is later killed by Shao Kahn, who uses the hundreds of souls Shang had consumed to empower Sindel.
- Goro - Prince of Shokan and former reigning champion of Mortal Kombat. He led his people's forces during the invasion.
- Kano - Former head of the Black Dragon criminal cartel in Earthrealm. He offered to aid Kahn's invasion in exchange for wealth and power, providing Kahn's forces with advanced human technology and weapons and instructing them in their use. He survives in both timelines and escapes back to Outworld after Kahn's defeat.
- Reptile - The last surviving Saurian who serves as a spy and assassin for Shao Kahn.
- Mileena - A hybrid of Edenian and Tarkatan created in Shang Tsung's Flesh Pits. After Kitana's defection, she becomes heir to Shao Kahn's throne.
- Kintaro - A Shokan gladiator and Shao Kahn's bodyguard.
- Baraka - Leader of the Tarkatan hordes.
- Sindel - Former queen of Edenia before it was conquered by Shao Kahn. She committed suicide when casting a magical ward to protect her then-infant daughter Kitana from Kahn's corruption, but was reborn thousands of years later in Earthrealm and bent to Kahn's will. In the original timeline, Sindel was freed from Kahn's control and would later take back Edenia from the emperor's grasp. In the new timeline, she killed most of the Earthrealm warriors herself until she was stopped by Nightwolf, who sacrificed himself to destroy her. Sindel would be reborn as a revenant under Quan Chi's control.
- Sektor - A Lin Kuei cyber-ninja, son of the clan's Grandmaster. In the original timeline, he survived and later fought Sub-Zero for the clan leadership, but was defeated. He fled to Japan where he founded a new cyborg army called the Tekunin. In the new timeline, Sub-Zero killed him in order to assume leadership of the Lin Kuei.
- Cyrax - A Lin Kuei cyber-ninja that hunted Sub-Zero in the original timeline, only to be defeated and badly damaged. He was later rescued by the Outerworld Investigation Agency who restored his mind and recruited him. He dies in the new timeline.
- Sheeva - A Shokan warrior assigned to protect Sindel.
- Motaro - A Centaurian who leads Shao Kahn's extermination squads. In the new timeline, he is killed by Raiden.
- Ermac - A construct formed by merging hundreds of warrior souls into a gestalt entity. After Shao Kahn's defeat in the original timeline, Ermac gains free will and joins the Forces of Light. In the new timeline, after Shao Kahn's death, Ermac offers his services to Mileena, until she is dethroned by Kotal Kahn.
- Rain - An Edenian warrior with the ability to manipulate water. Long ago, he betrayed his realm to Shao Kahn and served the emperor in hopes of leading Outworld's armies.
- Quan Chi - A necromancer from the Netherrealm who aided the Outworld invasion in the new timeline. Though he offered his assistance to Shao Kahn, he was secretly manipulating the entire event in order to allow Shinnok to escape the Netherrealm and conquer both Outworld and Earthrealm.
- Noob Saibot - A revenant created by Quan Chi from the soul of Bi-Han, the original Sub-Zero who was killed by Scorpion in Mortal Kombat. In the old timeline, he served as an assassin for Shao Kahn while secretly spying on the emperor for Shinnok. In the new timeline, Noob was destroyed when Kuai Liang, Bi-Han's brother, threw him into a Soulnado.