The Portal Stone is an ancient relic and treasure of the realm of Osh-Tekk, belonging to the royal family of the realm. The stone allows portals to be created that can let the wielder travel instantly between realms.
After Shao Kahn conquered Osh-Tekk, he allowed its emperor Kotal K'etz to keep the Portal Stone in return for his subjugation. K'etz gifted the stone to his son, the then adolescent Kotal Kahn, and tasked him with traveling to and surviving in Earthrealm.
Kotal would use the Portal Stone to travel to Earthrealm and after reigning as a god over a tribe of Mayans, returned to his father, though an argument between the two caused Kotal to keep the stone for himself.
Kotal would use it to travel to Earthrealm, until his loyal follower Erron Black stole it to use in his kidnapping scheme along with the Black Dragon. During a battle against the Red Dragon, Kano stole the stone and fled to save himself. Kano would later attempt to bargain with Kotal Kahn over the stone, only for the emperor to savagely reclaim the stone.
The Portal Stone played one last significant role, with Kotal Kahn having hid it on Shang Tsung's Island before battling Reiko, allowing D'Vorah to find it and use it to bring reinforcements to battle Havik and his possessed champions.