Onaga was the last Dragon King. He ruled the realm of Vaeternus. Vaeternus was the origin of centaurs, shokan, kuatan, vampire and dragon races. Onaga, as other Dragon Kings before him continued to unite other realms with his. But he was forbidden to claim the essence of any realm by force. He had to win 10 Mortal Kombat tournaments to get his prize. He eventually won the essences of some realms and finally when he decided to conquer the outworld.
But Onaga died in an apocaliptic event that destroyed the whole Vaeternus realm. But many of its races continued to live in Outworld.