A Soul is the life essence within all living beings of the Mortal Kombat universe and can be used for a variety of purposes. They are usually associated with the color green in physical appearances.
Use of Souls
A soul gives life to its owner and a sense of individual identity but within the Mortal Kombat universe, they have a variety of other functions and can serve as sources of power and longevity.
Shinnok and Quan Chi's dominion over souls can be used to resurrect the dead as undead Revenants. These Revenants' only purpose is to loyally serve and protect those that resurrected them. While they still somewhat have their original souls, they are under complete control of whoever claims them.
One very notable use of souls is the supernatural force known as the Soulnado. The Soulnado is a massive, green, swirling tornado of spiritual energy and tortured spirits that are trapped between the realms. Those who can harness or even absorb its contents are fueled with immense power. The Soulnado has also been used for malicious purposes multiple times, one example being Quan Chi's attempt at forcefully taking the souls of all Earthrealm inhabitants by activating the Soulnado with a sacrifice.
In the Form of Power
While the manipulation of souls is primarily met with malicious intent, there are warriors that use it for good.
One example of malicious intent, as Shang Tsung has demonstrated, absorbing others' souls not only allow him to extend his lifespan, but also gives him the ability to replicate the souls of his opponents in order to appear as them use their abilities. Another advantage this power gives him is an incredibly extended lifespan and supply of power. As long as Shang Tsung is feeding off of other souls, he will continue to live and grow in power.
Another warrior, a being who is known as Ermac, is an advanced creation of Shao Kahn's and a conglomerate of countless souls. With all these individual souls and their power put into one entity, Ermac has multiple unique traits such as telekinesis and a vast knowledge of Outworld and its inhabitants.
Similar to Ermac is the blind swordsman, Kenshi. Kenshi uses his ancient and powerful katana, Sento to guide and aid him in battle. Sento is also fueled by a great number of Kenshi's ancestors' souls. With this power embedded in Sento, Kenshi was granted with the ability of telekinesis, extremely heightened senses, telepathy and teleportation. Kenshi, however, uses these abilities for good and is sworn to protect Earthrealm.