“Prior to the Dawn War, when the Sithe were in the prime of their power, they discovered this Chaos Primordial. Some of the most terrifying and powerful Sithe experts set off to capture it, then imprisoned it within an enormous Sithe prison that was meant for imprisoning major powers. I once entered that place with my master and saw the ‘Flamewing God’ myself. Back then, one of the prison gaolers introduced it to us and sait that it was incredibly strong. Three Sithe Exalts had to work together in order to capture it.” -Whitethaw
'Flamewing God is a Chaos primordial servant that Ji Ning tamed by tricking and feeding him 'Verdant Azuresoul'. '-DE book 37 chapter 29
Ji Ning had acquired a verdant azuresoul from the beastworld, one which Autarch Bolin had transformed into a magic treasure. So long as a Chaos Primordial ate it, it would fall under Ning’s control and become his servant.
Chaos Primordials were the most terrifying lifeforms of the Chaosverse, birthed from the prime essences of the Chaosverse themselves. They were born with virtually indestructible bodies, and they possessed inconceivable levels of power. However, they were rather unintelligent, perhaps comparable to ordinary mortal children at best. They wandered the Chaosverse alone, and when they occasionally ran into a realmverse they would devour the entire realmverse whole.
As a result, quite a few realmverses had been destroyed by Chaos Primordials over the course of aeons! Alas, there was nothing that could be done; even terrifying Otherverse Lords would only at best be able to stay alive in the face of a Chaos Primordial attack. There was nothing which could stop them save the Autarchs.