Furies

The Furies are a virtually unexplained force within the realms of Mortal Kombat. They are mentioned in Mortal Kombat and Mortal Kombat II, but have not been mentioned since, aside of the non-canon Mortal Kombat (novel), which mentions Shao Kahn as being the master of the Furies. The act of winning ten straight Mortal Kombat tournaments upsets or unbalances the Furies, which consequently weakens the dimensional barriers enough to allow interrealm invasions. Based on this, they may act as a sort of dimensional balancing force. It is unclear whether they are sentient entities or just cosmic principles.

Trivia

In Big Trouble in Little China, which inspired characters such as Raiden and Shang Tsung, the Furies are referenced as ancient positive and negative energies that cause the universal stability in the cosmos. This is also referenced in Chinese mythos, in which the Furies are represented by the Ying and Yang symbol used in Daoist magic and alchemy.
In Shang Tsung's Mortal Kombat II ending, he is said to have turned against and defeated Shao Kahn, and with the emperor's armies at his command, finally unbalances the Furies, and brings an unrelenting invasion of demons into Earthrealm, dooming it to eternal darkness.