Netherrealm War

The Netherrealm War is an event from the Mortal Kombat video game series, taking place in Mortal Kombat 4 and the prologue of Mortal Kombat X. This event occurs shortly after Shao Kahn's failed invasion of Earthrealm and is made possible by the emperor's attempted merger of Earthrealm and Outworld. Although the merger was reversed, the barriers between the realms were severely compromised, allowing Shinnok and his Army of Darkness to escape from the Netherrealm and wage war against all realms, even the Heavens themselves.

Original timeline
With the barriers between realms weakened, it did not take long for Shinnok and his followers to escape from the Netherrealm. As his first act of revenge against the Elder Gods, Shinnok led an invasion of the realm of Edenia which had recently achieved its independence from Outworld. Edenia had not had sufficient time to recover from the centuries of Shao Kahn's domination, making it easy prey for Shinnok and his forces.

From Edenia, Shinnok launched his assault on the Heavens, slaying many lesser deities. The gods Raiden and Fujin managed to flee from the slaughter and travelled to Earthrealm, enlisting its strongest warriors to oppose Shinnok's menace. The conflict was ultimately brought to a close when Liu Kang, the Grand Champion of Mortal Kombat, fought and defeated Shinnok. Unknown to Liu Kang, however, his victory was made possible because Shinnok's magical amulet had been stolen by the sorcerer Quan Chi and replaced with a fake replica. With his powers weakened, Shinnok was once again cast down into the Netherrealm and the Heavens were spared from his malice.

Alternate timeline
Shao Kahn's failed invasion of Earthrealm had been masterminded from the beginning by Shinnok and Quan Chi in order to weaken the dimensional fabric between the realms, allowing Shinnok to escape from the Netherrealm. Two years following the Outworld invasion, Shinnok's demonic armies attacked Earthrealm, causing widespread chaos and destruction.

Shinnok's plan was to assault Raiden's Sky Temple and gain access to the Jinsei Chamber. By infusing the Jinsei with his own evil essence, Shinnok would corrupt Earthrealm's life force and slowly destroy all life on the planet. During the conflict, the fallen Elder God was assisted by the undead revenants of the Earthrealm warriors who had perished during Shao Kahn's invasion.

Shinnok's assault on the Jinsei ultimately failed when he was challenged and defeated in kombat by Johnny Cage. With Shinnok beaten, Raiden took the dark god's amulet and used it to imprison him. With their master trapped inside his own amulet, Shinnok's forces retreated back to the Netherrealm.