Dromar was one of the five Primeval Dragons. His name meant "adulthood" in ancient Draconic, and he was known as "The Banisher." Dromar was an enormous blue dragon that could breathe a cloud of disruption that threw people from him, shook solid matter apart and had an effect similar to an electromagnetic pulse on machinery.
Dromar was trapped deep in the ocean by the Numena Lowallyn, who created a treasure trove for the dragon that was so vast that it sated all of Dromar's desires, leaving the dragon with nothing to want and therefore no reason to live.
Dromar spent millennia inside his cave until Rhammidarigaaz began awakening the Primevals to aid in the war against the Phyrexians. Darigaaz, Rith and Treva fought their way past the zombies of the merfolk that had built Dromar's prison and then destroyed part of his hoard, which brought Dromar out of his coma and filled him with a renewed desire to control the greatest gem of all: Dominaria.
After Darigaaz sacrificed himself to break the circle of Primevals, and Rith and Crosis were reimprisoned in their magnigoth and tar pit, respectively, Dromar fled from the wrath of the Dragon Nations, whose minds had once been dominated by the Primevals.