You have been chosen to represent Earth in Mortal Kombat. Be warned. Although your souls are protected, your lives are not. I cannot interfere any longer. Your Earth is being governed by the gods of the Outworld!
These are the words of Raiden...
For centuries Earth has used Mortal Kombat to defend itself against the Outworld emperor Shao Kahn. But, Shao Kahn has become frustrated by the failed attempts at taking Earth through tournament battle. He enacts a plan which began 10,000 years ago.
During this time, Kahn had a queen named Sindel, but she died at a very young age. Kahn's shadow priests lead by Shang Tsung, make it so Sindel's spirit will someday be reborn, but not on the Outworld, but on the Earth realm itself.
This unholy act enables Shao Kahn to step through the dimensional gates and reclaim his queen. Thus enabling him to seize the Earth realm at last.
Upon breaching the portal onto Earth, Shao Kahn slowly transforms Earth into part of the Outworld itself.
Kahn strips the Earth of all human life, and claims all the souls his own.
The remaining humans are scattered throughout the planet. These are the selected warriors that must defend Earth once again.