Rhaegos is one of the legendary great golden dragons. He lived in the Dragon Mountains south of Rubia.
Rhaegos has been fighting with his nemesis, the great red dragon Zoralth for millennia but neither managed to claim victory. This conflict usually involved not only their followers but the locals of the region (see Feyburn). This conflict also prevented Rubia from expanding southwards. Some scholars theorized that the fight could have been ended long ago but the dragons are treating it as a game and actually enjoying it.
During the invasion of Rubia Feyburn was the only city to defy the Imperium as it was under the protection of Rhaegos. The Imperium has cast an Edict of Frost on the Dragon Mountains to kill both Rhaegos and Zoralth, but they underestimated the power of the dragons. In the end Rhaegos sacrificed himself to contain the power of the Edict and save the people of the Dragon Mountains.