The Dragon Medallion is a powerful artifact that enhances its owner's powers to a greater power level.
This artifact came into Sub-Zero's possession shortly after his victory over Sektor and his return to the Lin Kuei clan, later enhancing Sub-Zero's ice powers to levels he never thought possible. It appears to have changed his appearance, although there could have been other causes. The Dragon Medallion first appeared in Mortal Kombat: Deadly Alliance and later reappeared as a collectible Relic called "Sub-Zero's Amulet" within the Konquest mode of Mortal Kombat: Armageddon. The Medallion appears on both of Sub-Zero's alternate outfits in Deadly Alliance and Deception, and by extension appears on his main outfit in Armageddon due to re-using his Deadly Alliance alternate as his new primary outfit.
The powerful Dragon Medallion was worn by Sub-Zero during his battle against the Deadly Alliance of Quan Chi and Shang Tsung. While on their way home back to Earthrealm, his own former apprentice, Frost, attempted to steal the Dragon Medallion from Sub-Zero in her own plan to enhance her own powers and to become the new Grandmaster of the Lin Kuei Clan, but was overcome by her own freezing ability due to her lack of strength and discipline required to control the medallion's power.
It appeared in Sektor's ending in Mortal Kombat (2011) where Sektor killed his father (the Grandmaster of the Lin Kuei), who tried to warn him that wearing the medallion would give great power, but with a price, and becomes the new leader of the Lin Kuei.
In Mortal Kombat X, Sub Zero has a variation called Grandmaster where he Gains an Ice Clone that can be thrown or used as a shield while he wears an illuminated Lin Kuei medallion fixed onto the right of his torso.